There is no "magic number" of properties to own, everyone has different goals and financial situations to get to their personal "magic number".  

I'm a financial planner who works with real estate investors - we have a client who plans to renovate a Clinton Hill (Brooklyn) property that has been in his family for generations into a multifamily rental.

My advice: focus on building a strong financial foundation first save consistently, manage your credit, and educate yourself on real estate fundamentals.

What financial institution should I talk to?

Some other copy & paste advice:Recommend you first figure out the property Class you want to invest in, THEN figure out the corresponding location to invest in.If you apply Class A assumptions to a Class B or C purchase, your expectations won’t be met and it may be a financial disaster.If you buy/renovate a Class A property in Class D area, what quality of tenant will you get?

If your financial situation means you can buy a single property per year, or even a property every few years that is entirely okay.

I had to figure that out myself as well a couple of years ago haha as already mentioned, first things first, make sure you feel financially stable and secure. 

Very few are in financial positions where tax strategies should be prioritized in their real estate business outside of perhaps timing and tax consequences of selling real estate. 

.,) and use QuickBooks for everything related to accounting/financials.

You are making the decision to grow your financial portfolio in a great way.