You can set the maximum number of connections to the Server at the Site level.
Start the Administrator Interface and connect to the Server.
At the bottom of the left pane, click the Server tab.
In the left pane, select the Site you want to modify.
In the right pane, click the Advanced tab.
Select the Max concurrent connections check box and enter the maximum number of users you want to allow at any given time. If the box is cleared, the server will not restrict the number of users.
Click Apply.
Note
The Max concurrent connection toggle will limit the amount of "socket" (or low level) connections allowed by the server. When this limit is reached, any subsequent connection attempt will generate a socket or network error in the client (as if the server want even there). This occurs because the server refuses the connection entirely. For an anonymous FTP server, it is recommended you limit connections on a per-User basis (in this case, the User labeled 'Anonymous'), which will at least allow the user to partially connect before being told that the server is full or busy. This is a more graceful way of denying the connection.
Setting maximum transfer speeds
Setting maximum connections per User
Setting maximum connections per IP