Rob is correct you are better off with two access points. Also keep in mind that the access point may state that it can support 20 concurrent users, but the actual bandwith is shared so if you have 20 users connecting at 54mb your allocated bandwith (assuming that your connecting with a 100mb connection to switch) is still only 100mb. Be cautious with what\who is running wirless. you might run into a slow network and unhappy customers. it is recommened that only 10 connections per access point.
Agreed, I have seen plenty of issues when getting close to the 20:1 ratio when in a conference room type scenario (meaning, 15-25 people using one access point at the same time - generally all checking email/web surfing.)
10-15:1 ratio is your best bet. And don't forget to think about expansion - multiple wireless devices per person.
As a follow up to the previous posts (which I agree with... shoot for around 15 clients per ap in such a situation) keep in mind that with 802.11b, you'll only have 3 non-overlapping channels per AP. That in mind, 802.11b isn't well suited for high density environments like conference rooms. If possible, try and make sure that your user base is all 802.11a. I had a similar situation, and ended up installing four APs directly under the conference table (which, was it's own can of worms) each set to the lowest power setting, so cell size was quite focused. For extremely high density projects, you can look into pico cells, but Cisco usually doesn't suggest using pico cell mode without an expert to help with initial design and setup. I'd say 2 802.11a based aps at moderate power output should suit you well.
To anwser your question meggerman, it is 20 simontaniously current sessions, which would include data and voice. 20 is max 70 would be a dream but not practical for bandwidth allocation. 20 users weather voice or data is not recommended. I would try to add another access point to split the load.
ex: at our facility we have over 60 users out on the shop floor with desktops, roughly 20 users with laptops and 5 wireless phones, all in all 85 users that need to access the network at all times, (the desktops are wireless also) we did a site survey, came up with 12 accesspoints to do 100% coverage at a minimal 11 mbs. (11 mbs is what is required for wireless phones/voice) hope this anwsers your question.