How many of these are you allowed to have?

As far as I know, there are no limits to how many shops you can have. However, you get 3 free shops then you have to pay for them. The cost is 50K * the shops you already have. Example: I have 5 shops, to purchase a sixth would cost me 250K.

I have a very vague recollection that when they introduced the payment option for more shop that they may have put in a limit, but I really can't remember for sure and I can't find any reference to it.