Logically without further guidance it matches the v3/5 equivalent of 100 logins
And probably the URL length limit at 2035 is a issue to consider also.
I’d keep it to groups of 50, everything I’ve run never really tips over needing to look up 50 people anyway (in a short time). If you are consistently needing to look up large number of users in this way, you might want to open a dialog with Dev Success and see how they can better serve you since, if you are always looking up a lot of users, you are either doing something you ought not to be doing or there is a better way to do it
Requesting more than 100 might have some fun with the return object being large for you and/or taking little longer than usual!