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!
regards
mike g
regards
mike g