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.
That's not nearly as easy to answer.