This depends on the network topology. Do you do local ack or not? Also, what is the bandwidth of the STUN interface?
In general, only 7 PUs is supported on an analog multi-drop line. The maximum bandwidth for mult-drop line is 9600 bps. Some vendors may implement non-standard protocol to achieve a higher bandwidth. However, the limit is still around 7.
If you the STUN interface to provide clocking, over 100 PUs can be attached to a STUN interface. I have never seen more than 20 PUs in this scenario though.
There is no limit on max LU imposed by STUN interface. The limit is restricted by the number of PUs on a STUN interface (mentioned above) and the number of LUs per PU (255 LUs per PU).
As far as the response time, 7 seconds seems too slow to me. However, I do not know if this is normal. It depends on the number of transactions generated by 1000 LUs. We need a sniffer trace to capture the SDLC frames.