Which professional baseball player as opposed to MLB player opens it up to more complicated googling. I have not found tidy record lists for Japan but think the record was set by Wladimir Balentien in 2013 at 60.
He has 15 HRs in 559 PA in MLB prior to going to Japan Where he has sandwiched his record setting season with 30+ HR seasons.
This would also include I presume minor league and Mexican league records. Closest I could find for that was this which only includes records since 1962.
I think Tony Lazzeri hit 60 homers in the PCL in the 20s but they were playing a 200 game schedule at the time. In fact I think a couple of guys collected 300 hits while the PCL played a 200 game schedule.
When I was a kid, in downtown Sheboygan, there was Joe Hauser Sporting Goods. Joe hit 63 bombs in 1930 for the Baltimore Orioles. Then he hit 69 bombs in 1933 for the Minnepolis Millers.
Here's another one: Name a player who had more than 5 seasons with more than 100 plate appearances and had an OBP above .400 every time. (there is at least one)