That's kind of the point though, you start with 12+ grunts because they expect you to have to swap out all the time. Once you start doing resistance missions + have people in for bond or psi development, there's this constant in/out across your entire roster, which is a great change from XCOM1/2 where it wasn't that hard to just have a core team of 6-8 and then ignore the rest.
Great story though - similarly my first chosen random encounter had a lot of Lost adds, I think it spent the first 6 turns just fighting them hand to hand. The triangular us vs. them vs. them makes some maps, particularly city ones, quite interesting, especially because anyone is liable to toss a grenade and (1) spawn even more adds and (2) mess up the geometry. lol