They generally run in a mix 1 + 3 formation. In sets of 8 or 4. An odd 6 wagon set was in use, 2 gennys and 4 standards. The genny units are usually at the ends of the rake. When they were ordered the 24 wagons were to be formed up as an 8, 6 and 4 wagon formation, this didn't go to plan. Using 5 of the 6 genny units, leaving 1 genny and 5 unpowered as spares. Wagons 51001-006 are the genny units, 007-024 are standards.
There are pictures online of a formation at Limerick Junction, I think, with genny units mixed in to the middle of the formation but this was for a stock movement and not in use.