One lender told me for the 7th residential mortgage onwards (till the 10th one), I have to put down 30% minimum down payment and I must have 740 credit score...Is it true? If it is, what would be the alternatives? (Seller financing, commercial loan etc...??)

Sounds bogus. Either start calling around to other small local banks or use it as the writing on the wall to start using other forms of non-conventional lending (seller financing/commercial).