How do I know how much to plant? Check out this useful resource.

I came across this useful chart on the website of one of my favorite seed retailers, Johnny's Selected Seeds:

http://www.johnnyseeds.com/assets/information/vegetablecharts.pdf

It tells you how many seeds you need to buy to fill in a 100' row (or acre, if you garden on that scale!) and, most usefully, how much you can expect to harvest from that seed. For example, you need 170 broccoli seeds for a 100' row and the average yield will be 75 lb of broccoli. If you are trying to grow all your family's vegetables, that is very useful information.

Enjoy!