That's not nearly as easy to answer.
> Several studies have found that professional musicians have more grey matter (the neural tissue involved in thinking, movement and memory) in some regions than non-musicians.
Which you might need to visit an ineffective bypass to see that: https://archive.is/F67Gf
Bp;dr: Playing an instrument or singing, gives you more gray matter, memory and executive function, and a slower cognitive decline. Playing multiple instruments doesn't have a benefit
What’s interesting is that many of the best musicians play multiple instruments. The incremental effort to pick up a new instrument must be fairly small. I can’t tell you how many people I’ve met that play great guitar, standup bass, and fiddle. ( Banjo and mandolin seem just a little less likely to be included. )
I hope I get there some day! It looks fun to put down one instrument, pick up another and continue ripping.
Edit: appears to cut off article
It's a whole brain workout in my opinion. Plus the physical aspect.