"But if you have hit the limit, or you're close to it, the best solution — again, according to McElhearn — is to actually split your iTunes library into smaller chunks. Your library has to be under the 25,000 song limit for iTunes Match to work.
To split your iTunes library
Click on the File menu and select New, then select "Playlist."
Name the playlist "New iTunes library."
Drag any music you want to move from your existing library.
Click on your Mac's desktop to activate the Finder.
Click on the File menu.
Select New Folder. Name it "New iTunes Library."
Return to iTunes. Make sure you can see the folder you just created on your desktop.
Make sure the "New iTunes Library" playlist is selected, then click the first song in the playlist. It should be highlighted.
Type Command-A (Select All) to select all the songs in that playlist.
Click and drag them into the folder you created on your desktop.
In iTunes, click the File menu, select Library and select Export Playlist. Save the playlist to the desktop or somewhere else you can easily find it.
That will copy (duplicate) all the songs in your New iTunes Library playlist into that new folder. Move that folder to a safe location, such as inside the Mac's Music folder. It will also generate a .XML file that iTunes can read which contains all the metadata associated with those files — whatever ratings you've applied, play counts and other details that aren't actually embedded in the music files themselves."
According to Kirk McElhearn on MacWorld, it can cause unexpected problems:
“...when you hit the 25,000 track limit, all sorts of strange things happen. I’ve heard from people who’ve told me that uploads get wonky, that iTunes displays various error messages about connecting to the servers, and that, even after a number of tracks are culled, there can still be issues. Sometimes, even after getting a library below the 25K mark, it won’t upload new songs, even though iTunes Match says it’s updating. Sometimes it’s hard to download songs. And automatic updating can be unreliable.”