So I planned to make werewolf , but I'm need to know do you need to transform once in day or is it just optional .

Thanks for advcance. Ralphiee.

Optional, but it doesn't stack up if you don't use it.
I found it handy on an early level character but used it less and less as I progressed, the disease immunity is always handy though.

You don't need to transform at all if you don't want to. You can only transform once a day however, unless you get Hircines ring, then its twice a day. After you level high enough, the werewolf form will be weaker than you, but the resistance to disease will still be worth keeping.

It is optional, it is like a power and can only be used once per day unless you complete a certain quest for a certain daedra lord

Ok so I'll do circle missions . Thanks to you guys.