Hi Yuval,its the max user concurrent user supported by one AP when threre is simultaneous VOIP calls without call quality decreasing (with SIP or other VOIP protocol and codec).
About coverage it depending of your space propagation
VOIP clients refers to things like Spectralink handsets that use Call Admission Control / QoS to guarantee airtime, it doesn't apply to VOIP applications like Skype/Facetime/Google Hangouts or even Wi-Fi Calling, all of which were designed to cope with an unreliable network without strict realtime guarantees.
Be aware though that regardless of AP, supporting more than 30 simultaneous VOIP calls is moderately challenging for networking equipment and you may find that your upstream firewall/gateway isn't up to snuff either!
So if I have an open space area (no walls) with around 50 users that will do normal Wi-Fi usage like browsing / video / standard voice applications. Will one R600 AP handle it? What are the guidelines for choosing between R500/R600/R700?