Let's assume HNTR, that gives us 43 Souls and 5 Pain Elemental if the wiki is still correct and a tactical shotgun, THE shotgun to carry around.
At 5 shells per elemental, we will use 25. The souls an elemental spawns are mostly killed during a shotgun blast, but let's say each elemental manages to still spawn 3 souls we have to deal with.
43 souls + 15 = 58. That's another 58 shells at 1 per soul, this is calculated veryyyyyyyyyy large here.
Thus 58 + 25 = 83 shells used.
If my memory is good there are 7 or 8 stacks of shells in the level 7 * 8 = 56.
For a net loss of 27 shells before the agony elemental spawns.
It's clearly not as bad ammo wise as you make it sound, if planning on use shotguns at all, 2 stacks should be caried at all times imo. And it's not like there aren't power cells either. The level is very easy unless on AoMr, AoB can be tricky as well as some other angels.