To OP...in addition to character meals, there are many popular non-character meals as well. Things like 'Ohana dinner, California Grill, Rose & Crown timed with Illumination, etc. can all book up fast. There are many nights where it's hard to find any ADRs at the restaurants inside MK. If you go during a free dining period or go during a busy time around a holiday or spring break...many things that wouldn't seem as popular also book up.
Now, how many TS meals you need is really on you to decide. I like a TS breakfast and TS dinner, someone else may just want 1 TS per day, and someone else may only want a couple of TS for their entire trip. You will be hit with a $10 pp no show fee if you fail to show up to an ADR, so you need to really think about what you want before booking. Still, it's easier to book and cancel within the window than decide last minute that you want something that's hard to get.