I know that I can only deduct interest (or any expenses for that matter) up to my rental income, but is there a limit on the loan amount on which I can deduct my interest against?

For example, I read that a couple MFJ can deduct interest on a mortgage of up to $750,000.

Does that limit apply to investment/rental properties, such that if my primary mortgage and my investment mortgages are above $750,000, I cannot deduct interest on the portion that's above $750,000?