Split number into equal sized groups

number of groups := ceiling(total / max group size)
number per group := floor(total / number of groups)
rem = total % number per group

You will have rem groups with number per group + 1 and number of groups - rem groups with number per group.

EDIT: Example:

total := 50
max group size := 15
number of groups := ceiling(50 / 15) // 4
number per group := floor(50 / 4) // 12
rem := 50 % 12 // 2

2 groups with 13 and 2 with 12.

There are many ways of splitting the amount between groups. It all depends on whether the only factor is the number of groups or if there are any other factors. See:

    static void Main(string[] args)
    {
        List<int> list1 = Split1(48, 15); // result is: 15, 15, 15, 3
        List<int> list2 = Split2(48, 15); // result is 12, 12, 12, 12
    }

    public static List<int> Split1 (int amount, int maxPerGroup)
    {
        int amountGroups = amount / maxPerGroup;

        if (amountGroups * maxPerGroup < amount)
        {
            amountGroups++;
        }

        List<int> result = new List<int>();
        for (int i = 0; i < amountGroups; i++)
        {
            result.Add(Math.Min(maxPerGroup, amount));
            amount -= Math.Min(maxPerGroup, amount);
        }
        return result;
    }

    public static List<int> Split2 (int amount, int maxPerGroup)
    {
        int amountGroups = amount / maxPerGroup;

        if (amountGroups * maxPerGroup < amount)
        {
            amountGroups++;
        }

        int groupsLeft = amountGroups;
        List<int> result = new List<int>();
        while (amount > 0)
        {
            int nextGroupValue = amount / groupsLeft;
            if (nextGroupValue * groupsLeft < amount)
            {
                nextGroupValue++;
            }
            result.Add(nextGroupValue);
            groupsLeft--;
            amount -= nextGroupValue;
        }
        return result;
    }