I'm starting to accumulate some holds in preparation for building a wall inside the house. The wall is 8' wide and 12' high, so just under 100 square feet without any overhangs, volumes, etc. Obviously, the more holds I have the more variety available (and I plan on adding holds as I can), but is there any sort of "standard" for holds per foot to make it work-able?

As many as you can buy. I've kept adding more and more as I find good deals or holds I really want.

This should be your goal:

Steven Jeffery's home wall. Submitted By: Finn the Human on Feb 18, 2013

I started with 120 on a wall of the same dimensions as yours. I thought it was a good start. In fact, I havent added any except for foot chips.

Don't buy a bunch of the same holds from the same manufacturer. Also break-it down by type based on your skill-level based on the angle of your wall as 100 holds may mean 100 jugs or 100 foot-chips, one obviously a bit more useful then the other.

Starting out I would keep it to just having enough for a few routes and adding more as you figure out what is missing.

So maybe 2-3 ending jugs, ~30 handholds, 10-15 foot jibs. For the handholds it is all about your skill level but having some easier holds is a better way to start.

Keep an eye out for sales as those are a good time to add a lot of holds to your wall.

Good reviews on some sets here: climbingholdreview.blogspot.com/

2 per square foot...I don't like cluttered walls...

To me a home wall is about training for outdoor climbing, not a substitution for the outdoors...so I suggest focusing on a set of system holds, HIT strips are my favorite for upping my bouldering skills the fastest before a road trip, maybe a hangboard or some Rock Rings by Metolius, and then I'd set training routes around your typical red point ability. I set 2 below my limit, one at my limit and another a grade above my limit...I like to run laps on the first 3 and then spend the rest of the session working the project. Once I send the project and can start running laps on it, I take down the easiest route and set the next project in its place.

After setting up several walls at my last 3 houses, I noticed that if I just set random routes, I got bored with my wall and stopped climbing on it. If I use it strictly for training then it feels like a better investment for my time and I'm more willing to keep climbing on it regularly.

I also tend to buy a new set of holds when I set a new project route. I buy a set of my weakest grip and slightly smaller than my previous project route. This way I accumulate holds I'm more likely to use in the future instead of buying a bunch of random holds to start out with that end up not being as useful to help me reach the next grade or outdoor project.

+1 for everything Brad just said.

While I see Brad's point, I guess I should point out that there are (at times) 7 kids in the house ranging from almost 5 months old to 15 years. So the wall won't be for me alone, the kids will likely use it more than I will... At least that's kinda my hope!