20 tiger barbs will not fit into a 35gallon tank when full grown. Twenty babies seems even crowded to me. But if you wish to keep all 20 to full maturity, you're looking at 60 to 75 gallons to house all of them. Tiger barbs can get a nice round 3 inches or more if well cared for and fed properly.
Personally as well, as a side note, I would only get maybe five to six tiger barbs. You might love the fish now, but having 20 of the same fish in one tank is going to get boring real quick. Tiger barbs come in a wide variety of color species from the albino tiger barb (white and light orange), green tiger barbs (green shiney), red tiger barbs (bright cherry red), ruby barbs (black with red noses), and black tiger barbs (black with very little orange or red). Get five each of four different species to add a bit of spice to your tank. Tiger barbs are usually orange and black, I've never seen a silver and black tiger barb, so perhaps what you are looking at are not really tiger barbs at all.
Beware the painted albino tiger barb. If you see any tiger barb that is pastel blue, purple, an unnaturally bright orange, yellow, pink, or anything else, don't buy them. These fish have been artificially colored, a process very stressful to them as well as one that will fade over time.
~~Colesea