Nelson Cruz - 4 times, 2016, 2017, 2018, 2019. 44 was his highest in 2015
Why am I putting Nelly on his own page? Well, because Boomstick was a lot of fun to have on the team, and it was a surprise to see him keep hitting and hitting and hitting in Seattle, despite his advanced age. You have to remember that he was 34 when he got to Seattle, had broke 30 HR twice in his career, and there were questions about his production from a PED test failure earlier in his career.
It ended up being one of the greatest signings in the history of the Seattle Mariners. Cruz would hit a total of 163 HR for the M's while slashing .284/.362/.546 with an OPS+ of 148. Three All-Star Games, Two Silver Sluggers, MVP votes in three different seasons, and 414 RBI made Cruz a lot of fun to watch every time he got to the plate.
You could argue that Cruz was the last true slugger that the Mariners have had, that performed like he was supposed to. Haniger hit the mark once, and Teoscar is a scary dude at the plate. Maybe if he stuck around, he could put together a run like he did and Toronto and join Nelly with a 4-year run of 30+ homers. It's unlikely, but fun to think about. For now, Boomstick is the last true slugger the Mariners have had.