Most outsourced HR has a lower bound for number of employees. I've seen them as low as 4 but they really don't like that and don't make any money. You should switch to one as soon as you can. It's a silly amount of paperwork for each employee, but all of the administrative stuff they do (including getting you healthcare plans) isn't something you should be doing in house.

Even if you don't care about getting to outsource the administrative overhead, the healthcare alone may be worth it. Since they're negotiating as a very large company, they can offer significantly better plans than you would be able to by negotiating with providers as a small startup.

Is it covered internationally or just US?

This is why I want a national healthcare service. Let's all form one giant company, one big single pool, for health insurance purposes.

I'm assuming you're in the US, and this is not at all what the ACA does (which is the closest approximation to a US version of NHS).

First-hand experience here: Trinet is the worst of the worst, managing to slim down the HR experience to the absolute bare shittiest minimum. It's degrading to talk to their reps. A disaster.