anonymous user, Apologies for the delayed response.
For this scenario you only need one Azure SignalR instance. Also, unit is just a logical concept for how many connections you can have at the same time. So if your connection is no more than 1000 you only need one unit.
One Azure SignalR Service instance can be used by multiple applications. In the ASRS instance, applications are isolated by their application name and hub name. Please refer this discussion thread for additional details.
Kindly let us know if you have any further questions on this, we would be more than happy to assist you.