How much should I charge to bartend an event of 20 people?

Details: it’s for a work event for my boyfriend’s mom and I’ve never bartended an event so I don’t want to ask for too much. (I have bartended at bars and restaurants 8 years) she told me to just tell her whatever is reasonable to pay me and I honestly have no clue.

It’ll be an open bar and I’m almost positive she doesn’t want people to feel expected to tip. It’s from 5-9pm and in an event space in a condo complex. No drink menu, just people come up and order. She’s providing the alcohol, I don’t know what specifically so I’m guessing beer, wine, and liquor + mixer, but I’m capable of making actual cocktails.

I appreciate any input!!