Get the download count for all versions of a package

ConsoleApp17.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <LangVersion>8.0</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="NuGet.Protocol" Version="5.5.0-preview.1.6319" />
  </ItemGroup>

</Project>

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading.Tasks;
using Newtonsoft.Json;
using NuGet.Configuration;
using NuGet.Protocol;
using NuGet.Protocol.Core.Types;

namespace ConsoleApp17
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            var packageId = "Newtonsoft.Json";

            // 1. Find the latest search endpoint using the NuGet Service Index API
            // See: https://docs.microsoft.com/en-us/nuget/api/service-index
            var source = new PackageSource("https://api.nuget.org/v3/index.json");
            var providers = Repository.Provider.GetCoreV3();
            var repository = new SourceRepository(source, providers);

            var serviceIndex = await repository.GetResourceAsync<ServiceIndexResourceV3>();
            var searchEndpoints = serviceIndex.GetServiceEntryUris(ServiceTypes.SearchQueryService);

            if (!searchEndpoints.Any())
            {
                Console.WriteLine("Unable to find search endpoints");
                return;
            }

            // 2. Find your package's latest metadata using the NuGet Search V3 API 
            // See: https://docs.microsoft.com/en-us/nuget/api/search-query-service-resource
            var query = new Uri(searchEndpoints.First().ToString() + $"?q=packageid:{packageId}&prerelease=true&semVerLevel=2.0.0");

            using var httpClient = new HttpClient();
            using var response = await httpClient.GetAsync(query);

            if (response.StatusCode != HttpStatusCode.OK)
            {
                Console.WriteLine($"Unexpected response status code {response.StatusCode}: {response.ReasonPhrase}");
                return;
            }

            var content = await response.Content.ReadAsStringAsync();
            var results = JsonConvert.DeserializeObject<SearchResponse>(content);

            foreach (var result in results.Data)
            {
                Console.WriteLine($"Package {result.PackageId} has {result.TotalDownloads} total downloads.");

                foreach (var version in result.Versions)
                {
                    Console.WriteLine($"Version {version.Version} has {version.Downloads} downloads");
                }
            }
        }
    }

    /// <summary>
    /// The response to a search query.
    ///
    /// See https://docs.microsoft.com/en-us/nuget/api/search-query-service-resource#response
    /// </summary>
    public class SearchResponse
    {
        /// <summary>
        /// The packages that matched the search query.
        /// </summary>
        [JsonProperty("data")]
        public IReadOnlyList<SearchResult> Data { get; set; }
    }

    /// <summary>
    /// A package that matched a search query.
    /// 
    /// See https://docs.microsoft.com/en-us/nuget/api/search-query-service-resource#search-result
    /// </summary>
    public class SearchResult
    {
        /// <summary>
        /// The ID of the matched package.
        /// </summary>
        [JsonProperty("id")]
        public string PackageId { get; set; }

        /// <summary>
        /// The total downloads for all versions of the matched package.
        /// </summary>
        [JsonProperty("totalDownloads")]
        public long TotalDownloads { get; set; }

        /// <summary>
        /// The versions of the matched package.
        /// </summary>
        [JsonProperty("versions")]
        public IReadOnlyList<SearchResultVersion> Versions { get; set; }
    }

    /// <summary>
    /// A single version from a <see cref="SearchResult"/>.
    /// 
    /// See https://docs.microsoft.com/en-us/nuget/api/search-query-service-resource#search-result
    /// </summary>
    public class SearchResultVersion
    {
        /// <summary>
        /// The package's full NuGet version after normalization, including any SemVer 2.0.0 build metadata.
        /// </summary>
        [JsonProperty("version")]
        public string Version { get; set; }

        /// <summary>
        /// The downloads for this single version of the matched package.
        /// </summary>
        [JsonProperty("downloads")]
        public long Downloads { get; set; }
    }
}

zOutput

Package Newtonsoft.Json has 363187770 total downloads.
Version 3.5.8 has 371478 downloads
Version 4.0.1 has 104026 downloads
Version 4.0.2 has 72464 downloads
Version 4.0.3 has 56128 downloads
Version 4.0.4 has 35853 downloads
Version 4.0.5 has 144331 downloads
Version 4.0.6 has 19351 downloads
Version 4.0.7 has 269272 downloads
Version 4.0.8 has 261249 downloads
Version 4.5.1 has 338693 downloads
Version 4.5.2 has 19936 downloads
Version 4.5.3 has 32155 downloads
Version 4.5.4 has 72339 downloads
Version 4.5.5 has 74870 downloads
Version 4.5.6 has 1883480 downloads
Version 4.5.7 has 282482 downloads
Version 4.5.8 has 250974 downloads
Version 4.5.9 has 215814 downloads
Version 4.5.10 has 333492 downloads
Version 4.5.11 has 7611395 downloads
Version 5.0.1 has 1605050 downloads
Version 5.0.2 has 270262 downloads
Version 5.0.3 has 310856 downloads
Version 5.0.4 has 1820025 downloads
Version 5.0.5 has 558454 downloads
Version 5.0.6 has 2968931 downloads
Version 5.0.7 has 471055 downloads
Version 5.0.8 has 3091282 downloads
Version 6.0.1 has 2495051 downloads
Version 6.0.2 has 857469 downloads
Version 6.0.3 has 2272229 downloads
Version 6.0.4 has 18483146 downloads
Version 6.0.5 has 2997978 downloads
Version 6.0.6 has 4653944 downloads
Version 6.0.7 has 786859 downloads
Version 6.0.8 has 15272670 downloads
Version 7.0.1 has 12990520 downloads
Version 8.0.1 has 1343575 downloads
Version 8.0.2 has 4331554 downloads
Version 8.0.3 has 9875041 downloads
Version 9.0.1 has 37344517 downloads
Version 10.0.1 has 29783504 downloads
Version 10.0.2 has 28854458 downloads
Version 10.0.3 has 58162926 downloads
Version 11.0.1 has 18937622 downloads
Version 11.0.2 has 30864487 downloads
Version 12.0.1 has 27545793 downloads
Version 12.0.2 has 25201304 downloads
Version 12.0.3 has 3476765 downloads