The paired information is stored in the NVRAM, there isn't a limit for the number of the paired peer devices. In the A2DP sink demo, it stores only 1 paired information. You may refer to the audio_gateway demo which can store multi paired information.
For A2DP, it can only support 1 connection with audio streaming. For A2DP connection without audio streaming, we only tried 2 connections.
If I have two active connections (one of them streaming), are they affected by advertising?
The stack will process the logic. It will be OK to have two connections.