There's no reason for there to be "millions on apps on the Play Store". Literally no reason. It's likely 99% of users would entirely be served by the top 50,000 to 100,000 apps. For the other 1%, there is sideloading.
As far as how many app reviewers to hire? Well, Apple manages their store pretty well. Google has about eleventy billion dollars in cash, so I'm sure they could afford to hire a few people to do the job.