No turbo.I read somewhere about 8 pin sensors measuring humidity.

Water injection on an NA engine has more limited benefits. You get to keep your engine cleaner, a bit more power, but more added maintenance.

Stock MAF sensor only has 5 pins. Is there an aftermarket unit with 8 pins?

Correct our MAF does not measure humidity directly but does measure air flow volume which is altered by the humidity of the air (density).

I guess they were introduced in 2012. Whether or not they work in older engines --who knows? More water droplets per given volume means less oxygen per given volume as I understand. Not there yet with water injection. Note, that I am not after power, only smooth running, long term high mileage reliable..