So preservatives have a variety of methods of action and usually they work best against certain types of microbial burdens. So maybe one preservative is very effective at killing bacteria, but not mold. You use blends to give yourself “broad-spectrum” coverage against potential contaminants.
Including certain ingredients may also help or hinder your preservatives efficacy- ex clays, lecithin, and some other plant based materials are more likely to bring burden to your formula. Meanwhile glycols and surfactants can change surface tension which makes it harder for some bacteria to live. There are even more ingredients like chelators, or pH adjusters which don’t directly preserve but act as boosters in their own ways.
Suppliers will usually tell you a range to use their preservative. If you are making the blend yourself it is a bit of trial and error.
Hope that at least starts to answer your question