Jmix and in Spring Boot in general uses HikariCP for a connection pool.
You can set its properties in application.properties with main.datasource.hikari prefix, for example:
main.datasource.hikari.maximum-pool-size = 100
The list of available properties see in Hikari docs, just replace camelCase with kebab-case.
See also Spring Boot docs, but for property names use main.* (or additional data store name) instead of default spring.*.
How many connections should I plan for per user? For example, if I need to support 10 concurrent users plus additional stuff like async email sending, occasional quartz jobs, and some REST requests.
It depends on how often they acquire connections and for how long. For example a pool of 10 connections can easily serve 100 users if they execute short transactions not too frequently.
In your case, 10 connections should be enough.