I know that sql server express has not a user limit, and every application has a different way to load/stress the server.

But let's take "a typical accounting software", where users input some record, retrieve some data and from time to time they make some custom big queries.

May someone share its own experience and tell me which is the limit of users that can realistically use a sql server express instance in this scenario?

I am looking for an indicative idea, like (as an example):

"I had a company with an average of 40 users logged in and the application was working ok on sql server express, but when the users become 60 the application started to seem non repsonsive"

(please note this sentence is pure imagination, I just wrote it as an example).