Depends. Bug out, any more than you can reasonably carry with other essential supplies is wasted. Bug in, how big is your storage area?
Here's my situation, as I see it.
Most likely is natural disaster (for example, nearly half of our town burned to the ground a few years ago) and I'm grabbing the wife, kids, dog and basics. We're going to one of three places. CCW and a few extra magazines, maybe a rifle if it's widespread enough to cause civil disruption, everything else is locked and left.
Secondly is we're stuck. Pantry and cupboards full of food (plus fridge and freezer as long as power holds out) plenty of bottled water, enough to last a few weeks, maybe a month of regular use. I've got a basement full of ammo, but if I'm dug in and fighting off invasion, I'll be overrun. One dude against the mutant ninja zombie bears? I'm toast. Neighbors are all nice people, but not exactly the fighting types. Besides, the last time I was housebound, it was snow, ice and no electricity that was my biggest threat.
Last and least likely, there's some threat to hearth and home that requires able bodied men (hey, I used to be one a while ago) to sally forth and conquer. I'm going to rummage through the attic, drag out the battle rattle and use that stash in the basement 420 rounds at a time. 210 in mags, an extra bondolier and resupply at home if able. I usually give this scenario the most thought if I've recently watched Red Dawn.
That was a bit tongue in cheek, but I do not think it's "silly" to plan for unpleasantness. It's actually the smart thing to do. However, most people, especially us "gun types" tend to over-inflate the value of a firearm in case of emergency. It's never wrong to have extra ammo or magazines. However, having a bit less of those might mean the difference in having enough drinkable water, food, hygiene items, heat, tools, etc. I think you're better off having a few hundred rounds and several spare mags to throw in a bugout kit than to have cans upon cans of ammo but no place to go, no means to get there and no supplies to sustain you on the way or when you get there.
ETA: To plan on X number of rounds per caliber is a good way to spend money you don't need to. Plan on a CCW and one particular long gun being the "bad stuff happened" guns. Don't try to supply for every gun or caliber you own. You can't carry it, you can't eat or drink it and it won't keep you from freezing to death. Have enough water, food and essential items to keep your family for a few days, keep it in one place so it can be thrown in the car trunk and you're gone if needed.
. Katrina is given as an example of a real event and you have to think back to the people that assembled and provided their own security for their neighborhood having a much smaller quantity of ammunition per firearm than the Rambozo fantasies some entertain. That served as a sufficient threat to keep criminals moving past them and on to unprotected targets. Look at the people in Ferguson that protected their property with a single shotgun or rifle and no LBE weighed down with mags. Real events don't hinge on piles of stacked magazines. They hinge on cooperation of neighbors and coordination.
