Is there a limit as to what I can receive from Mom as payment?

You may also charge her reasonable room and board.

If she pays you much above the going rate in your area and she later needs to apply for Medicaid, the amount over the typical rate might be considered a gift.

Otherwise, Mom can pay you any amount she wants to and can afford.

I suggest you have a caregiving contract in place, spelling out what you do and what she pays you.