There is no point to do more than 12K even on my pc. I7 4930K 6-cores 3.9Ghz. Lags will slow mana harvest. With 10K monsters at the same amount of time you getting the same amount of mana than with 16K monsters.
Try 10 waves worth of mobs (9990 mobs). If that runs rune, then try 15 waves worth next go. If the 10 waves didn't work so well, then reduce it a bit, to 8 waves, or 5 waves.
Oh? But if the lag becomes huge, you can run it overnight(I've done it 5 times successfully, each takes 6k freezesecs) and at school/work(2 times success, takes 4.5-5k freeze) and basically run it 24/7 almost.