How often to have sex if you are TTC

In studies done on fertile couples, having sex EVERY day of your fertile window resulted in a 37% conception rate. 

Having sex every other day during your fertile window resulted in a 33% conception rate. 

Having sex just once during your fertile window, resulted in a 15% conception rate. 

As long as the male partner had no issues with sperm function, there is no downside to sex everyday. (ie, the sperm does not need that much time to "build-up" as is commonly believed.) 

Clearly there is an advantage to more frequent sex, but it is very much up to the couples preference whether they want that extra 4% by having sex every day. 

Source: ASRM