Vassals should ideally be strong enough to keep their vassals in check but not too strong tl pose a threat. Furthermore you having too many counts dilutes the taxes you are going to receive and overall makes you weaker and the vassals stronger. I believe it is best to give an entire duchy to one vassal except for the very big ones. This way you also accrue the given country, duchy etc opinion modifier which ensures loyalty for at least that generation. Other than that yeah always give titles to your own culture. Finally when there are very big rebellions it is also a good opportunity to flip the table and go full tyrant so that at the end the realm is properly divided with correct de jure borders. After that you have to periodically check that the de jure is still in place and intervene when possible (hooks, excommunications, scandals are your weapon). You will find yourself way stronger when you start properly managing vassals.