Anything beyond 16 items would be too many rows for Excel to display. 16 items = (2^16)-1 = 65535 possible combinations (rows)
The number of combanations of 18 items, taken 9 at a time

C(18,9) = 48,620

C(19,9) = 92,378

so you could get a few more and not exceed the 2003 row limit

But I'm not sure what you want running down the rows?

For ex for C(4,2) = 6, were you looking for something like

1,2
1,3
1,4
2,3
2,4
3,4

Paul