How many hands per month should a pro play?

It's all about money. If it's not about money, then play as long as you can as many tables you can profitably.
From professionalism perspective is about all about money.

Suppose you live in western countries(canada/us/europe etc.), you probably need to make at least 5-6k+.
Suppose it's 5000 for now, suppose you work/play 2 hours a day. You need to make 167$ a day or about 83.5$ an hour.
If you change the hours to 4 hours daily it'd be 41.7$ an hour.
If you want to make it like the normal 9-5 hours with weekends off. Then you'd be working 22 days a month 8 hours a day. You need to make 227$ a day, 28.4$ an hour.

Now let's say 100NL, say you play 6-8 tables, for simplicity sakes say you avg about 400hands an hour.
You need to make 7.1bb/100 if you work 22 days/8hours a day each month.
10.5bb/100 if you work 4 hours daily
20.9bb/100 if you work 2 hours daily.

So in reality. Playing 100NL fulltime doesn't even make you enough money as a pro realistically because these winrates are too high even for a "pro".
Maybe 400nl or 1000NL is more realistic.
And this is just like the bare minimum entry point who is even trying to struggle for a living.

If you want to make like 20k+ monthly, maybe it's easier to find "softer" games like live poker, private games, or those club apps with shady house and transactions etc. In those softer games, bb/100 goes out of the window, multitabling goes out the window as well. But hourly still exist and be able to calculated if you play enough.