The actual number of neighbors required to expand isn't rising and there's nothing wrong with your game. I'll try to explain using the example that you're starting at 20x23:
The first notice saying you needed 46 friends was for the 21x24 expansion. Once you've met the neighbor requirement for that expansion, you've still got to pay the 30 million coins to actually purchase the expansion and make your bakery larger. Whether you choose to expand right away or not, the game sees that you've met the requirement of adding 46 friends for the 21x24 expansion and it moved on to list the friend requirements for the next expansion, the 22x25. This keeps happening until you've added all the friends needed for all the expansions - again, this happens regardless of whether or not you're actually purchasing these expansions as they open to you. So, you're really just seeing the game notifying you that these coin expansions are now available for purchase; each time the neighbor requirement rises and you add more friends, you're unlocking a new coin expansion.
It can look a little confusing to keep seeing the number climb like that so I hope my explanation helped clear the issue up and didn't make it even more confusing. In case I explained it poorly, here are some links to threads that also discuss the issue and might answer your questions better: