It was easy for me. I hit my current number of bettas while working full-time, and suddenly had two hours or so of water changes to do when I got home every night. It made me realise that my hobby was quickly becoming a chore, and I haven't purchased any fish for some time now.
Recently I picked up a 450 litre tank to divide up into 14 bays for my males so that I can cut back on my water usage and half our electricity bill.
Plus it seems there's always something wrong with my bettas no matter how excellent the care I provide. Honestly, it wears thin after about the hundredth time and has put me off ever having so many of them ever again.
It does become an addiction, but sometimes you have to stop and think whether you are spending enough time appreciating what you have. I came to realise I was barely even looking at my fish in between water changes, and that's not why I got into the hobby at all.