Thanks to better UltraPeer features and more support of it by Gnutella agents, you will see less estimated hosts in your immediate horizon.
Don't assume that the host count is the only horizon you can reach: in fact the statistics window only displays you the hosts which have directly communicated with you by performing searches on one of your files.
Most probably, you're running as a shielded leaf node connected via UltraPeers which manage the bandwidth for you.
If you share less files, you will get much less hosts in your immediate horizon. This does not mean that you can't get a huge horizon with your bandwidth:
most (if not all) user agents will only see about 1 thousand hosts in their immediate statistics with the UltraPeer feature, meaning that you will reach about 40K-50K hosts for your searches.
Works are in progress to extend this effective search horizon.
For now, there is no accurate value for the horizon.
You can see the existing improvements by the number of search results you get now with a single search.
Now I get more than 2000 results for a single search, coming from about 1200 distinct hosts, despite I often have about 1000 host in the horizon displayed in the statistics. This is really a good feature because more and more users are running an UltraPeer-aware agent (such as LimeWire and BearShare), which saves much bandwidth on the network, improves its scalability, and maximizes the upload/download bandwidth and speed.
Add now the support for meshed downloads, and we are starting to get good experience with the Gnutella protocol, which now can compete with the defunct Napster in terms of reachability, searchability, speed of download.