After reaching the player level 4, you are rewarded with a new market expansion every second time when you level up (on every even level).
Before v1.2.2202 update (released 20th of June) the expansions were released on every level. Due to this your next expansion will only come when the number of unlocked areas matches with the current settings. I.e. at level 14 you should have 7 markets open, but if you leveled to 13 before the update you can already have 11 markets open. In that case the next market will open on level 24.
The markets where you can expand your business are always adjacent to open markets. They are shown with a violet marker that contains an open lock icon. A tip for the expansion is that you should analyse the different markets and check if they contain strategic businesses that you have already bought on some other market.