I have been reading that Russia has, in addition to their main tank force, 18,000 tanks kept in reserve that range from models such as T34, T54, T55 and T72s. The US doesn't seem to do this. Would an additional 20,000 tanks ranging from Shermans, M24 Chaffees, M26 Pershings, M41 Walker Bulldogs etc. give considerable strength to our army?
Not really. Modern tanks can eat an old tank for lunch, and truly not worry about return fire from them in the slightest. Their only concern would be running out of ammo to kill them.
Old tanks would fight old tanks or soft targets then.
They wouldn't survive long enough to get to the battle field. Russia kept all of the old tanks for use as barter for their proxy country's they used to be allied with, additionally, they had a tactic of trading armies for a short while, the theory being they wipe out your army while losing theirs, they then trot out their second army equipped with old stuff, but because you have nothing, they win.
The problem with that tactic is they ran in to the real world and our tanks are MUCH better than those they field, and their first, second, and even third armies would be toast. So now, they are trying to build one good army that can survive a battle with ours.