If you are
capable of multi-tabling, and there are multiple appropriate MTTs for you to play, you should be multi-tabling.
This is a no-brainer as it is essentially hedging your bets.
Tournaments are a long, slow grind.
When we consider the upper limits of our ITM/win rates when playing MTTs, it's obviously going to be in our best interest to have many irons in the fire.
The only exception is when we don't have positive expectations (i.e. when we are beginners) or if multi-tabling is overreaching with regards to our abilities, circumstances or setup.
When you have been playing long enough most decisions are pretty automatic/obvious so this allows us to play more tables as our confidence & ability grows.
If playing more then one table detracts from your game, you should focus on a single table for sure.
A bird in hand is worth two in the bush...