A question like this is hard to answer, simply because it depends on a couple. There is no "should" or scientific number to give for the number of times per week (or day) that married couples have sex.
Lots of things come up in our lives - stress, our jobs, school, kids, relatives, and more - and can interfere with our sex life.
I think if anyone is in a marriage and thinks they should be having more sex, they must talk to their partner about it. Sex is a fun, natural and healthy thing to do, and so it should be something that people feel comfortable discussing with their partner.
Now, if you want numbers, I think it's fair to guess an average of once or twice a week. It's going to differ from couple to couple, obviously, and for everyone it's different.
As far as 'should have sex,' again, it's up to the couple to decide what works for them.
I want to add that while sex every day of the week might not be possible, it is important for a married couple to stay intimate, whether it's a good night or good morning kiss, a hug after a long day of work, a "just thinking about you" text, or a pat on the butt to remind them that you still find them sexy and are attracted to them. There are many playful alternatives to having sex in a busy life when you don't always have the time for it.
A good idea would be to set aside a date night each week which gives you both a chance to cuddle up to each other, reflect on your week, and share in a bit of romance.
Hope this helps!