A few weeks ago it was 2. Argon happened 23 days ago; Muse Wonder was 11 days ago.
Not sure I'd put Google there in practice. And while Muse is an impressive jump from behind it's still not quite at the frontier either IMO.
I think the former definition is a bit more meaningful since it avoids issues with determining what actually constitutes "the best" model out there. In that case, Google has been a frontier lab for a long time, Meta arguably as well, and you're also ignoring xAI which has been competitive for a while now. And, obviously, there's a bunch of Chinese labs which would deserve the title as well.
But if one is being strict (and willing to accept generally accepted definitions/measures/etc.), then there has only ever been two frontier labs: OpenAI and Anthropic. They're the only labs to have had "the best" models at any point in time since LLMs became a thing.
everyone else mostly seems to have just decided to cheat on benchmarks to make their models look better than they actually are, and none seem to want to actually try to make better models.
As long as LLM's can be distilled, and all these "labs" (what a weird name) can distill one another, there are going to be as many as capital is available to be burnt on the topic.