Yes. Or at least hint at it, at which point someone will probably volunteer or let slip some information that gives you a rough shape of the causal chain, at which point you know where to dig and pressure further, and eventually convince someone to confess or get a warrant to be sure.

If the prosecuting side has a reason to care that much, it doesn't matter whether it's 10 or 100 people - in fact, if it's 100 people, the original source is in deeper shit because this is now obviously not just personal use, but distribution.