Financing above 4/10 houses

[Daniel B.]

I currently have 3 rental properties and own the property I live in=4 mortgages.
I have heard that it is difficult to get financing after the 4th mortgage, does anyone know exactly what is different after 4? Reserves needed? I have also head 10 houses/mortgages is another barrier, is it possible to get financing on an 11th house? Are the restrictions simply for FHA loans, or do they also apply if I can put down 20%?

Also, when calculating estimated returns/cash flow, part of the mortgage payment is going to pay down principal, should this be considered in estimating returns as you are building up equity for yourself? It does not appear to be from what I have seen on biggerpockets. However interest rates are so low and as I am toying with 15 year mortgages a big chunk of my mortgage payments would be paying down principal from the start, and it seems like this should play some effect when considering returns.
Any advice would be appreciated!

Thank you!