You can have 6 meadows of any size producing at any time. This is not related to prestige.

You can have a maximum of 5 workshops (Any size). These are linked to overall EC prestige.

Below is from the Equestrian Centre manual section 1.6.2:
"The number of workshops you can use simultaneously depends on the prestige value of your equestrian center:
Between 0% and 20% prestige: 1 active workshop
Between 20% and 40% prestige: 2 active workshops
Between 40% and 60% prestige: 3 active workshops
Between 60% and 80% prestige: 4 active workshops
Between 80% and 100% prestige: 5 active workshops
Production duration and required resources depend on the items you want to craft. "

There's a guide to increasing EC prestige and earning equus from your EC on my EC page if that would be of interest. It details how I built up my own EC.

How do you grow crops