I wanted 2, my partner wanted 3, medical issues after the first meant it stayed at 1.
However, now, a few years later neither of us actually have ended up with regrets or feeling bad about it. It is really nice that our son is growing up and progressing through stages, and we are already getting a bit more of our life back, which wouldn't be possible if more young ones were on the way behind.
I think as well, its hard to know until you have 1, whether 1 is ok... and 2, whether 2 is the best, 3, whether 3 is... etc. because until you have lived that reality, its just how you imagine it to be.
Personally I don't think you should focus on regret, because you can regret in either direction. There are certainly people who regret not having a 3rd, but equally you see stories of people on here who had another child and who then, by no fault of the child (and unfortunately often related to ND/SEN issues) ends up leaving the parents tinged with a guilty regret for having "messed up" the perfect family they had.
Things can go wrong in either direction, so you just have to take the action based on your heart and desire, and fully except that either way, you might end up loving the outcome, or not, and that is just part of what life gives us to deal with.