Hi Frank. This filter is listed as 16" x 25" x 4". Unless otherwise specified by the manufacturer (as in cases of spun glass filters) it is best practice to change an air filter seasonally or every 3 months. If you have extreme allergies, specific environmental concerns like wildfires or synthetic pollutants, or lots of pets in your home, then you should consider changing them more frequently.
This is the price for one single filter.