Since you control the triggers, you get to stack them how you want. So, you can put the Necropotence triggers (each card is a separate trigger) on the stack first and then the Memory Jar trigger, resulting in the Memory Jar trigger resolving first and all the Necropotence triggers resolving second.
Thank you, that's what I figured, except the part about each card beign a separate trigger. Afetr reading the oracle text today it makes sense.