The kind of camp you're looking for is often referred to as an "AFK camp" (because you can AFK in-between kills). Checking the
https://wiki.project1999.com/Per-Level_Hunting_Guide, it doesn't look like there are any "long spawn" camps for level 50 Necromancers ... but there are "AFK camps" (some of which are even marked as such in the guide; others aren't).
There are
lots of guard camps at that level (elves, dwarves, halflings, humans, trolls, etc.), and most if not all have safe AFK spots. If you want to use your Necromancer anti-undead spells, there's the Spectral Keepers in various parts of Trakanon's Teeth (terrible loot, but I got my cleric several "solo AFK" levels there).
Finally, there's a couple of island spots that would also work. In OOT you have Seafuries (you can AFK at a nearby island ... or if you have an EB item, just off in the ocean somewhere ... and they sometimes drop gems). In Timorous Deep there's the Spirocs: no gems or other meaningful loot, but the XP is good, and there are lots of places on Spiroc island where you can safely AFK.