Most of the pictures I have show one each end, sometimes two in the middle if the train was to be divided. I do not think the GWR used standard rakes of coaches for all trains like the southern did. The photos I have seen are mainly of trains headed for weymouth or on the dawlish coastal line so my knowledge is limited, I'm sure that others will be able to give you chapter and verse of the train makeups!
If you are talking about the grouping era, common practise to all UK railways in planned train formations was to place a guards brake at or near each end of a train of more than about four or five carriages. Most such trains effectively shuttled between the teminii they served, and by forming a train in this way there was always guards accomodation in rear: this was also convenient for working the train from the carriage sidings into the departure station.
So, you could quite reasonably include three brakes in a rake of - say- eight coaches or more : one at each end and one three from the end, to model a train that was going to be split somewhere along its journey.
The Guard's compartment (often referred to as the 'van' although it was not a separate carriage as it once would have been*) would be on the outer ends of the outer coaches. There were two reasons for this:
1. The guard would have all the train in front of him and did not have to look round behind to safely start the train;
2. in the event of one train running into the back of another there would not be many passengers in the back of the front train so casualties would possibly be reduced.
* There were 'Full Brake' coaches also - these were used on passenger trains where much luggage was expected, or used in fast parcels and newspaper trains where carrying capacity was important. There are ready-to-run models of these in 4mm scale; I don't know about 2mm which is what I assume you are modelling in.
There are quite a few books on coaches - it might be worth your while looking in your local library, or if their collection is catalogued 'on line', taking a look at that.
A forum community dedicated to Model train and railway owners and enthusiasts. Come join the discussion about collections, displays, models, styles, scales, motors, reviews, accessories, classifieds, and more!