It is recommended that both sides have the same number of units for the best balanced initiative. So Usually this means 2 elementals and 2 mechs versus 4 inner sphere mechs.
If you pit the 5 omnis and 2 elementals against the IS 8 mechs from AGOAC, the 7 clan units at base skill are about 12k BV, while the 8 inner sphere mechs are about 9k base. So you probably are gonna drop the gladiator or a lighter mech + 2 elementals, and have ~12-14 units total with 3 paper mapsheets--a pretty big game.
If you pit the box against itself, its the two big mechs (twolf and Gladiator) versus the 3 smaller mechs, with each side getting 1 elemental. Or you split the twolf and gladiator up, as they cost about the same, and split the nova+2 elementals and grendel+puma up, and that comes out about even as well. Of course, changing variants can alter the ratios a bit, but overall its pretty close.