A dating site is only worthwhile if other people are on it. Obviously it's possible, but you need people on it to attract more people.
Yes, you need a critical mass of people, but what this means can be very different for different people. Some dating sites have a warm body approach where you probably need lots of people because the set of matches is terrible.
However if you had a dating site for swedish people living in miami who only wanted to date swedish people even having 40 people on there might be plenty.