The practical limit is 26, unless you somehow hack windows to allow alphanumeric drive assignments. The physical limit is higher, but that will require swapping assignments as needed. However, you might be able to map drives to folders instead of drive letters.
OP, as long as they are self powered drives you won't have any problems. Also, multiple USB ports might belong to the same controller which mean shared bandwidth, so if you want to maximize transfer speeds make sure they are on seperate controllers. In the event you have only one USB 3.0 controller, it would be acceptable to use both drives on the one controller. .02