There are several problems here.
Firstly, your cbChoices array is not initialzed, so do `cbChoices = new ComboBoxnudGuests.Value;
Secondly, C# uses 0-indexing (starts counting arrays at the 0th element instead of the 1st), so use for (int i = 0 ....)
Thirdly, do you really need the array of ComboBoxes or can you just add them without storing them?
This should point you in the right direction.