It is definitely possible to have non-axon framework clients sending commands and queries through Axon Server. I have done some experiments using the npm grpc package to connect to Axon Server from node.js.
Apart from this, Axon Server also has a REST interface that you can use to send command/queries or to read/write events.
With regard to the maximum number of concurrently connected clients, it is hard to give any limits. There is no hard limit on the number of connections.
Normal services would maintain an HTTP/2 connection between client and server, your UI clients would probably not keep any open connections, so I would not expect limitations in that area.
The maximum number of clients would then depend on the number of requests and the time required for the handlers to process the requests. If necessary Axon Server could be run in a cluster (Enterprise Edition) to scale further.
I hope this answers your question.