Hi and welcome
Most of the frames have 20 slots in each row in a column, and there are 4 rows in one column - so 80 slots in one column. Usually each slot is for one letterbox, although some in some cases there may be 2 letterboxes/houses to a slot. I work on a van share that has 2 people delivering to 2 rounds - the frames on our 2 rounds both have 6 columns, that totals 480 delivery points on each round - 6 columns of 80 letterboxes. But a few slots are wider for businesses, schools or even houses that get a lot of mail. Our two rounds combined have around 930-940 deliver points. We take around 4.5 hours on delivery, on quiet days it can be done in under 4 hours but on busy days well over 5.5hours.
We sort and deliver letters, magazines, parcels (large and small) and unaddressed leaflets - there can be anything from 1 to 7 leaflets for each house and the first job each day is to fill a column of the frame with them - so with 6 leaflets that is 480 items put in before you start doing the post! Some of the letters arrive pre-sorted in the order that the delivery points are in - but that only applies to 'normal' sized letters, everything else has to be manually sorted.