Do you need 100 concurrent users (all users using the internet at the same time), or up to 100 users connecting at any given time (attached to the network, but may be inactive)? 100 concurrent users is going to require multiple access points (most are going to accommodate 8-25 users on average), so you would need at least 4 access points minimum....
Are the offices wired or wireless? If so, you may want a dedicated AP for the office.
Also - what is the area coverage for the devices? (how large is the building) Is this a metal building, or does it have major obstructions?
The more complex the building layout is, and the more complex with users, you may need to get a professional to do a site visit and at least give you a suggestion as to the layout.