Need help to solve the connection problem with our WiFi

So we have WiFi connection and it is being use by 100 users at once. I asked the ISP they said 50Mbps can support all of them. But I don't see it can. I check the speed and it is stable but surfing the internet doesn't seems to work correctly plus the device sometimes keep disconnecting from the WiFi.

Edit: Router model: Tenda ac1200 Number of access point: 1