If you have a Spellsinger Bard in your party you can actually boost it to 51 with songs.
I've done some thorough calculations and come to the conclusion that the highest Necro DC currently possible is 50, regardless of whether you are PM/AM, and can only be achieved by Human or Drow.
The +2 Drow Int Bonus won't get your Necro DC to 51 because you are without the +1 Enhancement Bonus Human will have. However being a Drow does make you having more choices when slotting your gear (if you are Human you are stuck with both a +7 Item and Litany).
Practically, 44 is about the maximum sane number you could get - and even that requires 1 TR. -4 away from some really rare and short buffs, -1 away from +4 Raid Tome (tough luck), -1 away from Completionist (nerds only).
Just FYI because yesterday on Amrath I heard some guy saying a "good" PM should have like 47 or so Necro DC, which is kinda silly (from a normal player's perspective).