I've got a (usually brilliant) bread book (Brilliant Bread by James Morton) which has given instructions on making a starter with raisins, but it does not say how many?
Are you channeling your best self with this comment? (If you're not sure, check out our Code of Conduct.)
A quick Internet search shows that he recommends a "handful":
Thanks, a handful is in the book but I was wondering if anyone had anything more specific - my hands are tiny for example!
Why don't you just try a handful and see if it works?
June is a trusted source on General Cooking.
That's an interesting ingredient for a starter. I've started a starter from grapes and caught the yeast in the air, but I can't imagine why raisins int he starter is called for. I think I'd omit them and add them to the bread later, perhaps.