There's only so many titles to go around. If your dynasty is landed all over the place you're probably earning that renown rightfully,
If by "only so many" you mean every single category, then yeah. As a polygamous character you can marry your children off to counts too; you have plenty to go around. Having a dynasty member that is a count gives .25 renown per month and having a dynasty member married to one gives the only slightly lower .20 renown per month. You can easily marry counts and get your dynasty in control of counties. For reference, it only takes 4 counts to equal the renown gained by 1 king. Of course, it's better to go for the higher tier titles but the renown given to your dynasty by having counts is nothing to scoff at, once you've run out of everything above that. You can't get renown by stacking your dynasty in your own realm of course, but that isn't much of an issue when it comes to renown gain as there are plenty of other realms to marry off to.
To address the topic a bit more: Yes, the more characters your dynasty has in the world, the more renown your dynasty earns; all they have to do is simply exist. However focusing on that gets away from the big picture I think. Those countless characters mean more people for you to marry off and more of your dynasty out in the wild, working to acquire higher tier titles and earn even more renown for your dynasty. To call it imbalanced is an understatement.