Glancing over the introduction page I see two spreads that are 5 cards each. I have two spreads in this ebook which are 6 cards and another one that is 7 cards.
In general, I think you will find that most relationship spreads are on the "larger" side, meaning at least 5 cards. Relationship spreads tend to have more cards than other types of spreads because there are so many aspects to cover - from fact finding aspects to advice. Usually you are going to need card positions for person a, person b, and the relationship dynamic between the two. And then, if there is a love triangle you have cards for that third person as well.
You can always take an existing spread and do a modified version of it by removing certain positions. You can also break it down into mini spreads.
Over in Using Tarot Cards there is a very interesting thread entitled How Many Cards Are You Comfortable With In A Spread?
that may interest you. In that thread I talked a bit about spread making and the issue of numbers:
"When I create my own spreads they are usually under than 10 too (I just glanced at my index and a lot of them seemed to be around 5). I put a lot of work into deciding exactly what I want each position to answer and I really try to weed out repetitive or unnecessary positions. However, I don't let the numbers game deter me from asking specifically what I want to know either so I don't worry if they go over that number."