It doesn't matter ...
too much.
I've been on Mastodon since 2016. I've switched instances twice, once when the instance was shut down (the maintainer gave several months' notice), once when instance ownership changed.
The first time was early enough that profile migration didn't exist ... and it still wasn't much of a deal. I follow a few hundred profiles, and am followed by about 2,000. It turns out that it's pretty easy to re-associate, at least for individuals. (Commercial / brand profiles with large follow/follower lists would have a different experience.) We reconnected pretty easily.
The more recent switch used profile migration. This brings along your followers / following lists, as well as block and mute lists, and is straightforward. Toots are not migrated, but for the most part Mastodon "lives in the present", and the back-catalogue isn't particularly significant. (I care more about this than most, and ... it's still not that big a deal.)
I actually have several active profiles, though all are forwarded to my primary active one: <https://toot.cat/@dredmorbius>. Again, if I need to fallback to another instance, I can.
Larger instances tend to be fairly anodyne. Smaller instances tend to fit niches, of both specific interests and policies. Pick something that looks reasonable. If you decide you want to change at a later date, it's not a big deal.