Array lists

public static List<List<object>> Split(List<object> source)
{
    return  source
        .Select((x, i) => new { Index = i, Value = x })
        .GroupBy(x => x.Index / 25)
        .Select(x => x.Select(v => v.Value).ToList())
        .ToList();
}

This will take your list of strings of file names and split them into lists of size 25.

Then I would use the TPL to create an array of tasks based on each sub list of the parent list and run the work from there.

Without more information I cannot write the task portion hope this helps you get started.