The game solves tracking this by providing counters for these units when in general reserve, and when in all three Russian armies. So you have 24 units in all, with the other army-designate counters being discarded once they are assigned. Assigned out of General Reserve is permanent. Sequence of movement is important. Relatively simple terrain types and effects.
Losses are in steps, marked off the record sheet.
Retreats are interesting. Don't have to retreat in the direction of supply map-edge so possible to retreat into a better condition. I managed to do that twice. Calvary are in general useless fodder for both sides. The battle is a bloody slogfest and much fun.