Connecting small networks

Each of your small office networks can easily be covered by a single AP, if all the users are clustered in a small enough area. Since you mention capacity, you may want to go with an 802.11a or 802.11g AP per office to provide more shared bandwidth. For example, 10 people sharing an 802.11a/b AP will receive an average 2-3 Mbps per user if they are relatively close to the AP.

You can learn about 802.11-compliant products by browsing the Wi-Fi Alliance website. Visit https://www.wi-fi.org/OpenSection/certified_products.asp?TID=2 and select the type of products you are interested in. Given the modest size of your office WLANs, you can probably consider less-expensive SOHO products from companies like SMC, D-Link, Linksys, and Netgear, although many large companies go for enterprise-class products from companies like Cisco and Proxim. If you are creating these small office networks from scratch, you might also consider combination SOHO firewall/access points from companies like SonicWALL and WatchGuard.

It is not clear how far apart each of your small offices might be, but typically you would connect separate office networks together using a wired backbone. For example, use DSL from each small office to a local ISP, and some type of VPN to interconnect those office networks together securely. If offices are actually in adjacent (or nearby) buildings, you may be able to interconnect them with point-to-point fixed wireless. Without knowing more about your topology and geography and business needs, I can't really point you towards suitable products. If you are interested in outsourcing your network access and VPN needs, you might want to check ISP-Planet's list of managed security (VPN) service providers. To learn more about fixed wireless, check ISP-Planet's How To Page.