Around 15-20 (which equates to £15 ish) items depending on availability at our community shop but every one that we've visited runs slightly differently!
We put items on colour coordinated shelves/in baskets depending on their value (we do purchase some things as well as recieving lots of donations and Fareshare deliveries).
Green they can (generally, again depending on availability) take as much as they want - fresh fruit/veg, short dated/past BBE items etc.
Yellow items are items that cost up to £1 (soup, beans, pasta, uht milk, biscuits etc) - generally we say 10 of these (if we've had to restrict green items or had a lot of donations like before Christmas we might say choose 12, or if we're running low or lots of red items, we might say choose 8). We will usually say no more than 2 of the same item, again depending on stock.
Red items are things that are more expensive (cheese, meat, ready meals, meal kits) and we usually say 2 or 3 of these (we actually said 5 of these a few weeks ago after some very generous donations so we were able to say 8 yellow items).
Hope that makes sense!