What About /Players settings?
Essentially, you want to minimize the number of other players in the game. In Single-Player, that means using /players1 exclusively. Do not run the Countess on /players8 if you want to collect runes. The reason is how the Countess drops items:
Without getting too in-depth, basically the Countess has two item-drop-checks. The first drop-check that occurs is a (mostly) normal item drop, which is why she likes to throw Town Portal scrolls all over the place when you kill her. The second drop-check that occurs is her special rune drop. Combining these two drops, the maximum number of total items she can drop at one time is 6.
If you increase your /players settings, it increases the chance that the Countess will drop more items in her first item-drop. You don't want her to throw out 6 different items during this drop because the odds of them being armor, weapons, gems, or whatever, is high - and the odds of receiving a rune from the first drop are minimal. If she drops 5 items from the first check, that means that she is only capable of dropping a single rune from the second drop check - and you want her to drop multiple runes.
And before you ask, no, there is nothing you can do to inhibit the first drop-check and encourage the second one. The best you can do is try to limit the number of items spawned in the first drop, so that if you're lucky with the second one, it will be able to spawn all of the runes it can. Hence /players1.