x 100!!Dual purpose is a bit of a misnomer...any bird can lay eggs and be eaten...'dual purpose' birds often do neither very well.
Depends on your goals, expectations, and housing/population capacities.
Do you plan on getting 3-4 new chicks every year for optimal egg production and eating the older ones?
If you're just starting out, get a mix of birds, see how it goes, keep researching, and after a year you'll know much more about what you want.