using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var records = new List<Foo>() {

            new Foo(1, 1),

            new Foo(2, 1),

            new Foo(3, 1),

            new Foo(4, 2),

            new Foo(5, 2),

            new Foo(6, 3),

            new Foo(7, 4),

            new Foo(8, 5),

            new Foo(9, 6),

            new Foo(10, 7),

        };

        var groups = Group(records, 3);

        for (int i = 0; i < groups.Count(); ++i) {

            System.Console.WriteLine($"Group {i}: {Newtonsoft.Json.JsonConvert.SerializeObject(groups[i])}");

        }

    }

    public class Foo {

        public int Id;

        public int GroupId;

        public Foo(int id, int groupId) {

            this.Id = id;

            this.GroupId = groupId;

        }

    }

​

    public static List<List<Foo>> Group(IEnumerable<Foo> records, int maxGroupSize)

    {

        var rv = new List<List<Foo>>();

​

        foreach (var record in records) {

            bool added = false;

            for (int i = 0; i < rv.Count; ++i) {

                if (rv[i].Count < maxGroupSize && !rv[i].Any(a => a.GroupId == record.GroupId)) {

                    rv[i].Add(record);

                    added = true;

                    break;

                 }

            }

            if (!added) {

                rv.Add(new List<Foo>() { record });

            }

        }

        return rv;

    }

}