Can I sign up for all currently existing StackExchange sites, or am I supposed to sign up or create accounts for ones I'll be active on?
You can do whichever you prefer. You'll have to sign up for each site individually and there's no particular benefit to having an account on a site you don't participate on, so in my experience most people don't create an account on a site until they need to. But "need" can be anything from asking or answering a question to flagging something to favoriting a question. I once created an account because I wanted to bookmark (i.e. favorite) a question on their meta.
Accounts that have 1 reputation and no participation get automatically deleted after a while (six months? a year?). So if you create such accounts and never use them you mind find that they've gone away, but if so you can just recreate them. The association bonus protects you from this.