From what I've seen, Necromancer staples are:
[mwcard=MW1J18]Poison Gas Cloud[/mwcard] (2-4 copies)
[mwcard=MW1J20]Sacrificial Altar[/mwcard]
[mwcard=DNQ05]Libro Mortuos[/mwcard]
[mwcard=DNQ02]Death Ring[/mwcard]
[mwcard=MWSTX1CKI01]Drain Soul[/mwcard] (1 copy)
[mwcard=MW1J11]Idol of Pestilence[/mwcard]
as well as the host of poison curses in the dark school.
As for skeletal army composition I think you want to figure out how you want to win and what tactics you want to shore up against or power through. Let me elucidate.
If you want to advance with a never ending horde, then yes, you should have 6 skeletal minions, 4 death fangs, 4 sentries...etc. In that instance, you are hard countered by [mwcard=MW1J21]Suppression Orb[/mwcard] and [mwcard=MW1J16]Mordok's Obelisk[/mwcard]. You better have a way to kill these things quickly. On the other hand you get a ton of mileage out of [mwcard=MWSTX2FFJ04]Armory[/mwcard], [mwcard=FWE06]Fortified Position[/mwcard] and, of course, [mwcard=DNC07]Mort[/mwcard].
Contrast that with the "Few Big" or "Buddy" mentality and you could build a book around [mwcard=MWSTX2FFC10]Sardonyx, Blight of the Living[/mwcard] or a few [mwcard=DNC14]Skeletal Knight[/mwcard]s In this case you'd want to support them with [mwcard=DNI03]Reassemble[/mwcard] and positional cards. Maybe even some attack spells. You get around the obelisk and the orb this way, but you don't have an army to stand between you and the enemy and might want to pack some more equipment than usual.