Online sign up system that tracks quantity needed remaining

I've used signupgenius.com in the past and it's ok - does what I am needing in that I can set up a list of items needed and as people sign up the quantity adjusts down until all items needed are supplies. Their service is a a little clunky to work with. Is there anything that can do the same thing that's a bit more modern (and ideally free for small numbers). Seems like this is a niche that could use a few more options if someone is trying to figure out what they should create next! It seems most of the bigger form builders I am familiar with don't offer this capability of tracking the remaining amount needed. Thanks in advance. Jon