For the academy? Where I would assume players start with us from ages of around 6 upwards? Quite a few.
Also, having more than a couple gives you the opportunity to train defenders, midfielders etc. seperately.
In fact if you look at the plans above it lists the uses of the pitches - Under 9s/10s, U13s, and over 18s. One pitch is unlabeled though which I would assume is for the U18s.