Unfortunately, the client does not know how to process a number larger than 8 for transportation sizes.
If you hack and bypass it, then the game client will be all wierd and the new slots will end up in some wierd place.
This is a limitation from WC3, and you have to accept that T.T you can try shift + double clicking the field and it will allow you to change the value past 8, but the client has minimums and maximums that it goes by. Wait till SC2 maybe T.T
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.