Generally speaking, there's no recommended number of people per ad. The quality of your audience is more significant compared to your audience size. You should consider your campaign objective to determine if you have enough audience as your audience size will follow your overall goal.
But, you can check the 'Audience Size' on the right-hand corner
of your Ads Manager to check if you have a broad or specific
audience. As much as possible, you want to be highly
targeted.