You should always have a few accounts in reserve, but if you don't know what you are doing and open to many, you might get them all linked and kill them all.
It will all depend on how long your accounts last. If you are selling high risk vero items like me and proably alot of other people on this fourm :P you will need alot of back up accounts. The more accounts you have the better imo
I have 4 active accounts atm and i have another 10 in back up which are are all aging nicly =)