Question
how many seeds of sweet pea should i plant to grow about 500 gram ?

Answer
Dear Farh:

Sweet pea seeds typically come in single packets of 225 seeds (Gurney's) and if you get 80 percent germination, the crop should yield more than 500 grams.  See the description on the link below:

         http://gurneys.com/little-marvel-pea-/p/14822/

Best Regards,

Mack Jean
Master Gardener
Tennessee USA