Synodontis species range in sizes up to 1 foot in length, the upside down cat is one of the smallest. How you select the species should be gauged according to space in the tank, and how large of a tank you will end up in.
I don't promote overstocking a tank and then "thinning them out later" because many times there is nowhere to go with the extra fish. (unless you're planning to upgrade to a large enough tank when needed)
Most LFS's won't take in many fish anymore, they are hard to relocate and can get expensive to care for until a home is found. Africans breed quite easily, so the higher the stock in a tank, the more issues you invite later when the fish mature.
Before stocking too heavily, you may want to contact your LFS to be sure they are willing to take the fish you plan to get rid of. If you don't, you may find you will get stuck with them, and end result, you'll need to purchase a larger tank or deal with the many issues of an overstocked tank, including dead fish.