You can display up to 294K tags at the same time, but such a large KML file usually leads to performance problems and sometimes causes problems.
The KML test generator was having serious problems loading a single KML file with 500K marks. A similar test with 100K marks had a long launch delay (~ 2 minutes), but after that it worked smoothly. These numbers provide a rough order to the limit in a single KML file.
Suggest breaking the labels into something like time chunks, and then having the parent KML file using network links to download all the chunks. In NetworkLinks, everyone must have a TimeSpan for their respective time segment. Then only segments (segments) that are active will be downloaded and displayed. If performance is still a problem, then you can break it into smaller pieces.
The root KML will look something like this:
<kml xmlns="http://www.opengis.net/kml/2.2"> <Document> <name>Parent Document</name> <open>1</open> <NetworkLink> <name>Group #1</name> <TimeSpan> <begin>2012-08-12T01:00:00Z</begin> <end>2012-08-12T01:59:59Z</end> </TimeSpan> <Link> <href>group_0100.kml</href> </Link> </NetworkLink> <NetworkLink> <name>Group #2</name> <TimeSpan> <begin>2012-08-12T02:00:00Z</begin> <end>2012-08-12T02:59:59Z</end> </TimeSpan> <Link> <href>group_0200.kml</href> </Link> </NetworkLink> ... </Document> </kml>
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