When my dealer friend received his shipment of 500,000 Federal SP primers last year I re-stocked. I think I'm down to about 50,000 of them right now, plus another 20,000 Federal LP primers. That doesn't count the S&B, MagTech and Winchester primers I keep in stock, but I do a lot of loading and shooting. I'm also holding 10,000 SP and 10,000 LP Federal primers for a friend of mine from out of state who will be visiting this summer, and can't find them where he lives. Plus, we don't have any sales tax......
The bottom line is to keep as much in stock as you can afford. I try to keep at least a four year supply on hand at all times, especially primers. Without them, powder is just fertilizer and bullets are just fishing weights......... When you find them, buy them.
Hope this helps.
Fred