I like to have the ability to reload every magazine at least three times. Or at least a case of ammo per shooter (one each, pistol and carbine). Which ever number is greater. This is the advice I to people who ask for my advice on the topic.
As for myself, for the last several years, when I buy ammunition, it's at a 2/1 ratio. I buy twice the amount I expect to shoot. The new ammunition goes in the rear of the shelf and the older stuff goes in the front. Over time, it's not hard to build up a respectable amount of ammunition.
It's much cheaper to buy by the case. And cases stack so nicely.
Lets just say that my kids will be shooting some of my ammunition when I'm gone.
When my kids were young, they could go through a brick of .22lr in an afternoon of plinking. So every payday, I'd swing by Walmart and pick up 2-3 of those 550 round value packs. I was paying around $7.25-7.50 per 550rd pack. My supply of .22lr would need a larger selection within your poll.
Within the last 10-15 years, we've all seen periods where ammunition in the common calibers (9mm and 5.56mm) dry up and become difficult to find. People standing in line at 6:00 am at Walmart waiting for a truck to be unloaded so they can fight over a 50 rd box of 9mm WWB FMJ. When the war on terror started, even agency contracts were backlogged 12-18 months.
While I would never recommend going into debt to buy ammunition, but I would strongly recommend stocking up now if you have the means and if your inventory is less than what you would like it to be. Price and availability is one the side of the buyer right now.