If by connect to a transmitter you mean receive a signal from a transmitter then you can have almost any number of receivers receiving signals from the same transmitter. As to how many,that depennds on the transmitter power and range. Consider a broadcast transmitter, the number of receivers 'connected' can be many millions.
If you are sending a specific message to each receiver then the number will be limited by the time it takes to send each message and the cycle time between messages.