I don't know of any hard limits, though there may be some practical limits depending on how the devices are used. For the database auto compaction to happen, all the devices need to periodically be synced together, and that might be a bit inconvenient to make happen. I don't think it needs to be a showstopper, however, as you can force the compaction to happen manually if necessary.
You do need to be careful about how you use this system. Everyone will be using the same database, and it is a "last one in wins" system, so if two or more clients change the same item, the last version of the change to be synced is the one that will be kept going forward, and there is no warning of a collision, or logging of who contributed the change. It really isn't intended as groupware at present. If everyone is going to have their own folder of projects and never do anything but look at other people's projects, this may not be as much of an issue, but even then I'm not willing to bet that there won't be some bumps. I think your odds of success would be higher doing this with 8 people that really understood the program and how the sync works, and with 8 people that didn't quite get it, Murphy might make an occasional appearance.
How much data are you planning to have in this system? It has been suggested that about 1,000 actions is a good target to stay under for best performance, and if you have 8 independent users of this database, that's an average of only 125 or so. The system shouldn't suddenly fall over dead at 1,001 (Omni's CEO mentioned at one point that he had 1200+ actions in his without undue difficulty on the iPhone), but the iPhone client will be working up a bit of a sweat as the database grows :)
If you are using the Bonjour syncing, you'll want to be sure that the machine hosting the database (one of the desktops) is always on and running OmniFocus or no one will be able to sync.
As for discounts, you'd be eligible for the standard discount -- each license you buy after the first, for any Omni product, all in the same purchase, knocks 5% off the total price of the purchase, up to a 30% discount. So you'd pay 5*$79.95*(1-(.05*4)) = $319.80 for the desktop part. I'm not exactly sure how bulk licensing for the iPhone client works, you might only have to buy one client if you sync all the iPhones to the same iTunes account. Email
sales@omnigroup.com for definitive answers on this stuff.