I keep a rough running inventory with an average number in mind and the number of rounds I buy or am prepared to reload at a time. When I hit around 50% of the average number, I start casting around for good deals.
For example, my average number for .40 S&W is 1,000. When I hit 500 I will buy another 1,000 primers, 1,000 bullets etc. for the reloading and maybe 250 factory rounds if I am going to be short brass. So I'll usually have between 500 and 1,500 rounds on hand give or take 100 or 200.