First of all,sex outside marriage is sin.But for married couples,sex is the basic foundation of every marriage,because we go into marriage to produce children which involve sexual practice.
In the process of conception,couples can do it almost everyday because they know what they are seeking for.
But in times of pleasure,sex can be done,whenever they are in the mood for that because sex involves two people and it must be enjoyed by the same two individuals,therefore it doesnt matter how many times it should be done,what matters is the pleasure that is derived from it.