First, you'll need to add additional fields to the forum_polls table, following the same pattern as the ANSWER15, COUNT15 currently provided. To do that, you can either manually add them, or use something like this. It's untested, but should work.
"pollchange.asp"
Look in the following files and change instances of 15 to the desired quantity:
config.asp
Dim strHidePoll, strVResults, PCount, nCount, txtAns(), vPercent(100), vAnswers(15), vCount(15)
active_polls.asp - all
admin_poll.asp - all
inc_poll.asp - all
pop_poll.asp - all
post.asp - those related to poll mod (should be seven)
topic.asp - those related to poll mod (should be twelve)
admin_poll.asp will take a bit more work, sorry. You'll need to expand the <select></select> for strMaxVotes or replace it with a input type=text.
I redid this whole thing on my modular to use a new variable in lieu of the 15 throughout. That way it can be changed in the admin page and not have to edit all the pages each time.