Bell peppers it's generally good to work on 1/8 bell pepper per piggy per day. So you'd probably use one and a half peppers a day roughly, so about 10 or 11 per week for 13 piggies.
For cucumber I give between 2-4 slices to my 2 piggies, so 1-2 slices per piggy per day. 1 decent size cucumber easily lasts a week between 2 guinea pigs. Youd probably need more like 1 a day for 13 piggies.
Haven't given celery yet so cant comment on that.