CountBug.cs

using System.Collections.Generic;
using NUnit.Framework;

namespace NUnit_CountBug
{
    [TestFixture]
    public class CountBug
    {
        [Test]
        public void CountConstraint_ShouldWorkForInterface()
        {
            Assert.That(new[] { 1, 2, 3 }, Has.Length.EqualTo(3), "Length"); //ok

            /*this obviously will not work. It's a shame though that you do not provide non-breaking
             "Cardinality" constraint that encompasses both Length and Count*/
            //Assert.That(new[]{1,2,3}, Has.Count.EqualTo(3), "Count"); 

            //this works
            var list = new List<int> {1, 2, 3};
            Assert.That(list, Has.Count.EqualTo(3), "List");

            //this too...
            var readOnlyCollection = new List<int> { 1, 2, 3 }.AsReadOnly();
            Assert.That(readOnlyCollection, Has.Count.EqualTo(3), "ReadOnlyCollection");


            
            IReadOnlyList<int> arrayCast = new []{ 1, 2, 3 };
            //so far so good
            Assert.That(arrayCast.Count, Is.EqualTo(3), "ArrayCast without constraint");

            //crashes here even though Count exists
            Assert.That(arrayCast, Has.Count.EqualTo(3), "ArrayCast");
        }
    }
}

NUnit_CountBug.csproj

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

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.0" />
    <PackageReference Include="NUnit" Version="3.12.0" />
    <PackageReference Include="NUnit3TestAdapter" Version="3.17.0" />
  </ItemGroup>

</Project>