Maximum number of subscription possible by paho-mqtt python  library.
Normally subscriptions are held by the broker, not the client. The broker just forwards messages to the client that match a topic pattern and the client passes that received message to the callback.
How those topic patterns are stored will differ from broker to broker, but assuming even the most naive implementation of an array then the limit would likely to be the size of an int on the platform the broker was running on, which is likely to be larger than any sensible system would ever hit.
If the client library is keeping track of the list of subscribed topics (which I don't believe the Paho libraries do as there is no need), then the list is likely to be on the same scale as the broker.
Also be aware that you can subscribe to wildcard topics, this would hold a single slot in any list, but could match any number of actual topic a message is published on.
what is maximum number for subsciption can be handle by paho- mqtt