Would work 10-15 simultaneous Wi-Fi connections with different devices: smartphones, laptops, tablets
If the traffic is not great - easily any mid-range camera. Say Mikrotik price categories: around $ 200 cope hurrah.
If the traffic is great - not right no device. Well this is physics: devices interfere with each other.
The correct version - if there is not enough speed to put on 2-3 different Wi-Fi channel with non-overlapping frequency.
To wifi network normally operate not only within the office but also within other on the same floor, remote not more than 50 meters from the router
This depends on the wall material and power antenna, etc. or At least there are repeaters Wi-Fi....