In case for a SharePoint list a lot of list items are needed for testing (eg. for threshold problems), this script could be used. Be sure to use the correct fieldnames in the for-loop. The items will be commited in batches, to speed-up the process.

BulkAddListItems.ps1

param (
    [string]$Url,
    [string]$List,
    [int]$NumberOfItems,
    [int]$StartAt = 1,
    [int]$CommitAfter = 100,
    $Credentials
)

if ($Credentials -eq $null)
{
    $Credentials = Get-Credential
}

Connect-PnPOnline -Url $Url -Credentials $Credentials

$list = Get-PnPList -Identity $List

$endAt = $NumberOfItems + $StartAt - 1

Write-Host -ForegroundColor Yellow "Will create items starting at $StartAt and ending at $endAt"
Write-Host -ForegroundColor Yellow "Commit after every $commitAfter items."

$commitNo = 0

for ($i = $StartAt; $i -le $EndAt; $i++)
{
    $userId = "{0:10000000}" -f $i
    
    $firstname = "Firstname$i"
    $lastname = "Lastname$i"
    
    $username = "$firstname $lastname"
    $usermail = "$firstname.$lastname@whereever.com"

    Write-Host "$userId -- $username"

    # $item = Add-PnPListItem -List $list -Values @{ Title = $userId; Username = $username; Usermail = $usermail }

    $lici = New-Object Microsoft.SharePoint.Client.ListItemCreationInformation

    $item = $list.AddItem($lici)
    $item["Title"] = $userId
    $item["Username"] = $username
    $item["Usermail"] = $usermail
    $item.Update()

    $commitNo++

    if ($commitNo -ge $CommitAfter)
    {
        Write-Host "=> Commit"

        Invoke-PnPQuery

        $commitNo = 0
    }
}

Write-Host "=> Final commit"

Invoke-PnPQuery