We have a room which is not very close to the AP and the signal is low. But a laptop with a quality radio connects perfectly. I am wondering if this will change when the number of clients grows? Such as, lower signal, lower bandwidth available?
It doesn't affect the number of clients that can associate, but the lower the signal strength, the lower the "speed" (data rate). The lower the data rate, the longer it takes for a client to send a given amount of data. When that client is sending or receiving, no one else can be on the channel. So your overall throughput goes down.
The association limit is set at the driver level by the chipset vendor of your Access Points radio. If there are certain clients at low RSSI, it would inevitably cause issues with throughput and fairness, since Wi-Fi is contention based the lower signal of 1 client could compromise on the overall aggregate performance. Generally if you are using for low intense apps like streaming or browsing, you would not see any degradation in that, but you would see a marked decrease in let us say speed test throughput of this one single client in low RSSI if more client are associated and remain at higher RSSIs.
Your LAN/WAN bandwidth would remain the same, that is a flawed terminology. But the time your get to use this "bandwidth" would vary on your system.
Another factor is if your WAP has bandsteering enabled, based on RSSI, or PHY rate or number of client on you radio your client could be steered to a better band by the radio, provided both SSID's and security standards are the same.