There is not an easy answer for this question. There are so many variables that I don't think I can answer you in a way that isn't going to prompt more questions.
If you are expecting less then 20 people at a time in your form then I wouldn't worry about it in the least. However if you are expecting 20+ continuously you may want to consider SQL Server instead.
Having said that you may want to check out this site which boasts these numbers (as of when I posted this):
49 active users
2990 visitors today
5098 pages today
The site is
Cover Your ASP. Although I couldn't find the link when I ljust looked for it I believe the entire thing runs of an Access Database.
I have also seen
Snitz Forums which can run on Access handle 100+ people fine without any noticable degradation in performance.