Do I need a permit if I wanted to have a group picnic of a total of 12 people ?

Yep

Yes

On the Glenolden Borough web site it states, "Permits are required for Ball fields, Gazebo, Picnic areas and Organized Events." There is nothing defining the number of participants in the Picnic area. You can contact the Borough Office at 610-583-3221 between the hours of 8am - 4pm, Mon - Fri for additional information.

That's the poster policy although it's not always enforced.