The silversides also come in different sizes. To answer your question, when my snakes were smaller, they'd get about 4 each per serving, now I give them more...7 or so, sometimes more.

The good thing about silversides is that they are a complete body, where as tilapia or salmon filets do not contain the eyes, scales, brains, etc. Still, it would be preferable to add some variety to the diet (or supplement).