Is it basically as many as you want? I started off as an Archer, then when reached Level 15 started a Conjurer (thinking they specialised in destructive magic). I have now accepted to be part of the Arcanist Guild.
So can you join as many as you want?
Yes, thats how it is. Just be prepared to fill your armory with lots of tools, weapons and such
(For myself, I keep being the tool myself to save armoury space j/k)
I hope you notice you can borrow some tricks from your other classes?
Like "protect" or "cure" from conjurer can be mighty good to have playing other classes