4 units and under can typically get long term debt. 5 to 8 units looking at small commercial banks with shorter term loans.The small units take up a lot of time to operate and work for the projected yield.How much is your time worth is the question?

Reg A allows the issuer to advertise, meaning that they could recruit investors from the general public via marketing, and the regulators want to protect unsuspecting investors from unscrupulous operators--so a limit on how much those investors can invest might protect some folks.But the Reg D 506(b) exemption prohibits advertising--meaning that it is generally thought that investors aren't cold leads from a billboard, instead they are people who might have a pre-existing relationship with the sponsor, or is friends with other investors and so on. 

.This is going to be hard on the bigger operators with very large apartment complexes in Oakland--I wouldn't want to buy a Rent Roll in a Class A, B or C area consisting of unknown cut throats and tough to deal with residents.Plus I wouldn't want to have my property managers quit due to not being able to deal with hard to handle types. 

I am new in RE investing and would like to know is there any rule for percentage how much the mortgage payment should or should not be, and how much the operating expenses should be?

I own/operate a family owned residential and light commercial contracting business.

Many of these markets are very well represented by sellers & turnkey operators here on BiggerPockets.

I own and operate several AIRBNBs and know lots of people who do as well.

I have been in the Oil & Retail Connivence industry with Operations for the past 25 years & my wife is an RN who also doubles as Super Mom.  

Interest on a loan is part of your operating expenses.  

An of course we will want to get another house later down the line.An LLC may be your best bet with a really good operating agreement.