Multiple clickbank accounts? whats the advantages compared to 1?

Some affiliates I know have more than one because they've chosen to open a second (or subsequent) one(s) when the sales credited to their original accounts have dramatically and suddenly dried up for no apparent reason. In each case, on doing so and changing their affiliate links accordingly, the sales have immediately jumped back to about their former levels. There's absolutely no explicable or justifiable reason why this should work (and some people, because of that, even refuse to accept that it's true!), but it often does (and it's definitely true).

Yes, you can have as many as you wish, as an affiliate, and there's no charge for opening them. You still have to satisfy the "customer distribution requirement" and are still subject to potential "dormant account charges" on each separate account, though.

For vendors, subsequent accounts cost $30 each (technically a "product-activation fee", as you say, and not an "account-opening fee") rather than the $50 for the initial account.

Vendors can have up to 500 products within one account, but they're limited to one marketplace listing (and to one commission-rate) for all their products, if they choose to do it that way, so many don't.