How many number of user can use canvas app simultaneously in the published mode?
This is the best I could find. The main thing to consider is this, your user count WILL affect the performance of your datasource.
Solved: Re: Number of Concrrent Active Users for External ... - Power Platform Community (microsoft.com)
Is there any Microsoft documentation on this?
It's my understanding that you can have up to 500 concurrent users for a published Canvas app.