There are an unlimited number of skeletons in most, for there are spawners.

Did you ever see those pink/purple props in black forest, with greydwarves meeting around? This is the same in burrial chambers, for skeletons.
The strategy is also the same: locate and target the spawner first to avoid fighting a large number of enemies... or let them stand, and fight as many ennemies as you want, to collect loot and train your fighting skills (both as character and as a player).
Given the number of chambers in game, you should really need but one such training room, so choose one wich is convenient, and with manageable spawning rate and difficulty (not all seem to behave the same, but I may be wrong). In particular, it's best when the spawner is far away from entrance, with a door that can seal the area: you can leave the training session and close the door without triggering too much skeletons. Remember that they can break open doors if they sense you.