In modern use, pumpkin patches are farms in the countryside that merchandise a significant amount of pumpkins.
Originally, a pumpkin patch was the area of a garden where the pumpkins were planted and tended. The word "patch" has been used to describe a plot of land going back centuries
Pumpkin patches can be as big as a tradesman wants them to be. Only some of the pumpkins are grown at the site itself. Often they are purchased in large quantities and marketed on-site.
Pumpkin patches are usually opened to the public in the months of September and October when Halloween celebrations are at its peak.
In modern use, a pumpkin patch typically describes a farm or establishment in rural areas that stocks a large amount of pumpkins in the month of October.
It's a very American tradition to visit the pumpkin patch each fall, although it's more and more common for families to visit make shift patches on street corners or get pumpkins from the local grocery.