I wouldn't be too worried about the quantity of toys it's the quality of the experience that matters most. Whenever I host playgroup for my young sons, I find the kids to be very destructive. Especially the boys. This is no reflection on their mom/dads parenting skills, at all. This is about the fact that they are young, and very active children. I purposely lock up most of our best toys before the kids come over and put out toys that are fun, but if they get destroyed my kids or my husband (because the cost of things) won't be traumatized later.
So out comes the *generic* wooden trains (not the high buck Thomas), out comes the cheap plastic dump trucks, lawn mowers, and playground balls (not my kids' favorite Tonkas and Matchbox) and I preselect some good games and have crafting supplies on hand that a group can play and do with little parental involvement (in case I can't get to everyone or have no help). This is usually plenty for the kids, because their primary goal is to run! Their second goal is to tackle! Their third goal is to make a ton of noise! No one seems disappointed to play with the old and cheap stuff.
I also always plan a couple of easy to do crafts that don't make too big of a mess, using supplies you already have on hand. Our boys and their friends are crazy for Star Wars/Clone Wars stuff. I found a bunch of free crafts you can print online, and they loved it. Masks, coloring pages, 3-D ship and people/robot/creature models, and more. Just google star wars crafts.
Sometimes I plan an easy to make recipe (keep food allergies in mind) and have the kids put it together, then eat it.
Some games we have that are always a hit with the boys when they come over include:
Hyperdash
http://www.amazon.com/Wild-Planet-46001-Hyper-Dash/dp/B00...
Hide n' Seek Safari
http://www.amazon.com/R-Games-###-###-####-Safari-Monkey/...
What's In Ned's Head?
http://www.amazon.com/Fundex-2460-Whats-Neds-Head/dp/B000...
Beware of Dog
http://www.amazon.com/University-Games-53102-Beware-Dog/d...
Gassy Gus
http://www.amazon.com/Gassy-Gus-Board-Game/dp/B003DKUF1S/...
I love this book for game and activity ideas with groups of kids. It's a life saver for things like playgroups and birthday parties. See: http://www.amazon.com/Great-Big-Book-Childrens-Games/dp/0...
Some of our best playgroups are when I let the kids just play in the backyard on the wooden playset/jungle gym. There's no prep needed..,.and you can just pull up a chair and watch! If you don't have one, it is well worth the investment! My oldest is 10 and still loving our play system.
Prior to our wooden one, we had a metal swingset...that the boys did love. But they wanted a clubhouse, so we switched to a wooden set a bit later. We've had no regrets, though they can be pricey. But don't discount the more affordable old school metal swing sets ...when a large number of kids come over they are a lifesaver. You can get one for anywhere from $100 to $250 (with all the bells and whistles) brand new at Toys R Us. And you can find some for less or even free on sites like Craigslist. Walmart, Sams Club and Costco are a great place to save money on an actual wooden playset with clubhouse and more if you're up for making a more significant investment. Once again you can also find these on Craigslist, the only downside is you have to move it and reassemble without instructions or a warranty...so sometimes labor nixes the good prices.
see:
http://www.toysrus.com/product/index.jsp?productId=3424132
http://www.walmart.com/ip/Adventure-Play-Sets-Atlantis-Wo...
If you don't want to do that, or can't, just get or make a couple of soccer goals, get some playground kickballs, a croquet set, bocce balls(lawn bowling) some whiffle balls and plastic bats and have them do drills or actually play a game.