I like to use a mix of paid and free. It's easy to swap free ones out of a spam event happens or if your client allows you to configure global to be only paid relays.

Most users will be okay using the relays that s client gives you, because relay management is a new social feature that people have never had to do before. They'll need to add one or two that their friends use though.

With relays, I've found that less is more. Somewhere in the low teens seems to be best, with no more than 20. At least from my own experiences.