The most important rule here is, don't mix your anonymous and real identities. Don't send a public key that you use on SR to people who know your real identity. Don't post that public key on clearnet where it can be linked to your IP address. Beyond that, you can have as many public keys as you want identities, but most people maintain either one or two identities. In the case of two identities, they keep their marketplace and forum identities separated.