controller.rb

class PostsController < ApiController
  def index
    posts  = policy_scope([:api, Post.all])
    paginator = Paginator.new(posts, params[:page])

    render json: PostSerializer.new(
      paginator.paginated_relation,
      meta: {
        total_count: paginator.total_count,
      },
    )
  end
end

paged_strategy.rb

class PagedStrategy

  def initialize(params = nil)
    parse_params(params)
    # raise on invalid values here
  end

  def apply(relation)
    offset = (number - 1) * size
    relation.offset(offset).limit(size)
  end

  private

  attr_reader :number, :size

  def parse_params(params)
    case params
    when NilClass
      @number = 1
      @size   = default_page_size
    when ActionController::Parameters
      params  = params.permit(:number, :size)

      @number = params.fetch(:number, 1).to_i
      @size   = params.fetch(:size, default_page_size).to_i
    when Hash
      @number = params.fetch(:number, 1).to_i
      @size   = params.fetch(:size, default_page_size).to_i
    end
  end

  def default_page_size
    25
  end
end

paginator.rb

class Paginator

  def initialize(relation:, page:)
    @relation = relation
    @page     = page
  end

  def paginated_relation
    strategy.apply(relation)
  end

  def total_count
    relation.count
  end

  private

  attr_reader :relation, :page
  
  def strategy
    @strategy ||= PagedStrategy.new(page)
  end
end