Do you mean wordpress forum accounts? Do you mean wordpress.com accounts? Or do you mean the account you use to manage the site through the WP dashboard?
If the former - no, but you could if you wanted to.
If the second - probably the same answer, unless I'm missing something.
If the latter - each website will have its own account structure - it's like saying "do I need a new mailing address for my summer beach home?" It just comes with the territory.
Unless I'm not understanding the question accurately, in which case please feel free to provide some more info. Happy to help further if I can!