Purchased character slots are account-wide. If you've got 5, you have 17 slots on the destination as well. All 17 characters should transfer without buying more.
That's cool, but what if I get to the new server and want to buy another slot for a character? Will I be able to? Someone stated a while ago that they were able to move all their 16 characters but were barred from making any new ones until they had gotten the numbers down to below 11. Deleting characters is not an option for me