What amount should my mother pay as "rent" and how should I go about asking her.

The amount of rent would be whatever you and your mother agree upon. It's not unusual for a parent to help out with the groceries and/or utilities but it is always better when the parent makes the first mention of wanting to help out.

How long has your Mom been living with you? If Mom just moved in, and hasn't offered to pay for anything, starting the subject will be difficult. Hinting might help, like if Mom what certain food items when you grocery shop, quietly say sorry I can't afford to buy all those items, my budget don't stretch that far.... then see if she offers to help pay.