We usually see issues with using consumer equipment start at around 15 - 20 users. Consumer hardware is great for home networks, where fewer people have relatively low bandwidth requirements. Consumer gear often uses a mesh system, which exacerbates issues in a work environment since the access points have to communicate with each other as well as with all the clients. You can expect network performance issues around 10 - 12 people. Unreliable WiFi will significantly affect productivity, so we recommend being proactive by setting up enterprise WiFi in advance of that.