How many Peanuts In the shell = 1/2 cup?

That would depend on the size of the peanut. One idea would be to shell peanuts into a half cup measuring cup as you count each peanut opened.

If you can also log/measure your peanuts by weight, that would be the most accurate way instead of by volume, because the amount of peanuts per volume varies depending on the size of the peanuts.

How many peanuts in peanuts in the shell? [by weight]

Bought a 5 pound (80 Oz – 2.27 Kg) bag of peanuts in the shell and had it weighed
It weighed 5.12 pounds (US measure)
The bag with all of the empty peanut shells weighted 1.52 pounds.
Giving a weight of 3.60 pounds of peanuts or about 70% peanuts 30% shells

Shelled all of the peanuts and put the shells back in the original bad and took it to the store to have it weighed

I have had college physics and chemistry classes so I have some idea how to perform experiments.
One dish being used to collect shells overturned and all that could be collected was collected but there was some visible peanut dust that had to be surrendered to the carpet sweeper. [that is there was some small loss of peanut shell dust – estimate that we lost less then one whole empty peanut shell in uncollectable dust]

Also three staples of the type used to hold papers together were used to seal the original bag with the now empty shells in it. [that is there was some small additional weight – three staples]

I took reasonable care to get a reasonably accurate reading.

Other information: It took one hour to shell the first pound [was not rushing but did try as if I was doing it for someone else and they were watching - The US federal minimum wage is $7.25 per hour effective July 24, 2009. [This was posted Feb 16, 2011] So add in 5 hours worth of labor to shell the peanuts. [$36.25 labor $5.99 peanuts in shell for a total cost of $42.24 for a cost of $11.73] [Your time is worth something – that is how much it would cost you if you had to pay someone to do it for you] However most of the time we use peanuts in the shell we are doing something else; watching a game or video, reading or talking with friends. During those times the shelling time is free. Enjoy – the cost is only $1.66 when you are not paying for the shelling time. [the five pounds sold for $5.99]