You can just drag as many tracks into Picard as possible, cluster them, try let Picard auto-match them with 'Lookup' or 'Scan', and hit 'save', but the results wont be perfect. Picard isn't magic, and if something in the database is wrong, or existing tags have wrong information, it will make the occasional mistake/ you may lose information. Especially on so many files! So tagging less files and checking the results before saving is more accurate.
Back up your files first, or test Picard on a test copy
To answer your question, personally I wouldn't do more than 1k files at once, Picard can do more, 10k might do ok, depending on your computer. You can always do more but you're going to have to leave it running for a few hours to process everything, and it's going to look like it's frozen because you're going to run out of memory on your compy.
Download it here, it's only 9MB, start it up, add some files and see for yourself.