I always buy them when they are buy one get one free, even if I have some already. My pantry has anywhere from 8-16 at any given time. Have to make sure you've got all the flavors covered!!
I use mixes for my practice cakes. I try to keep 4 of each flavor I use in stock at all times.
Theresa 
I use to keep alot of them, different flavors and such. Now a days I don't keep more than a few chocolate and vanilla on hand. I also only do cakes here and there, so when I have a cake to do I end up going to the store anyway. I use to keep many different flavors on hand, but after throwing out many out of date mixes I quit doing that.
i stock up when they're on sale. Just last week i found DH for 80 cents each so i now have 15 white, 10 chocolate, and 6 yellow. I use white cake mixes as bases for other flavors too so i usually have more of those on hand. and with going into the holidays I know that i'll use more.
Now, i do like to always keep 6 white, 6 chocolate, and 2 yellow on hand, but get more when i find a good deal.
i would buy 3-4 cases (36-48 ) at a time. If I got down to 6 of one flavor, that was considered "out".
In the shop, I created a "minimum inventory level" list that I posted so my girls could let me know when I needed to restock. For example, I wanted to ALWAYS have two of the 7-lb bags of p.sugar on hand, so when I got down to 2, they'd let me know. Eggs - 2 dozen; 6-lb cans of Crisco -2; margarine - 10 lbs; food safety gloves - 2 boxes, etc.
(Learned thsi from my warehouse managing days when I had to control inventory ... making sure I had enough on hand and being able to give a restocking order to the plant with sufficient lead time.)
Now I think the record is held by a fellow CC'er I know who bought something like 150 or 250 boxes of cake mix. she found a fabulous price and stocked up!!!
Quote by @%username% on %date%
%body%