We usually see issues with using consumer equipment start at around 15 - 20 devices. 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. Unreliable WiFi will significantly affect productivity, so we recommend being proactive by setting up enterprise-grade WiFi in advance of that.