If you plan on using your oven for 6+ hours / 5 days a week (or more) then 3 or 4 layers of blanket will be beneficial. If you are like 98% of our customers and use your oven for about 4 - 5 hours at a time, then 2 layers will be more than adequate.
If you are building the 40" or 46" Mattone Cupola dome oven, we recommend 3 or 4 layers of Ceramic Fiber Blanket since ovens that size are normally used for a little more than the typical backyard event.