@Gidon Levy this entirely depends on each person individually. Preferences, experience, energy level, appetite for risk etc.
Investing in one specific market is what many people do, and it works great. I also know of investors who have land holdings in 10+ states, and they also do just fine.
An argument I would make for investing across different states/markets is it spreads out your risk. For example, say you own 5 properties in a city, but 2 major corporations move out of town to relocate elsewhere. Demand drops, renters move out and it becomes more difficult to adapt with all your eggs in that one basket.
As I'm developing my strategies for investing, I want to go for a diversified approach. This means investing in multiple markets and multiple different unit types. This is MY preference however, this may not work for you.
As long as you have a solid 'Core Four' in each market (Agent, Lender, Contractor, Property Manager), you can invest anywhere.
Hope this helps! Reach out anytime if you need help.
-Andy