More Nest Wifi units isn't neccessarily better. Too many placed too tight might degrade performance. This is both due to interference, and to the extra overhead added for every additional unit.
About how many is enough:
There is also a new firmware version rolling out that could make things better, should hit all Nest Wifi Pro by the end of this week.
How to check what you currently got:
You missed the point, either for lack of understanding or intentionally. Too many Nest Wifi units too tight can cause worse wireless performance, and thus worse mesh connection status.
Do what you wish with that information, but it's hard to give any further advise if you don't want to play ball.
Like, tell us how big your place is, what your walls are made of, and maybe draw a simple model of your house and Nest Wifi pucks placement.
About reconfiguring all your devices when you switched to Nest Wifi Pro: If you reused network name (SSID) and password, you wouldn't have to.