I have one wonderful fortress that has several large extended families, if I count inlaws. I decided to lump them into clans by that criteria. With about 200 Dwarves I had 17 clans ranging from one to half-a-dozen married couples and their children, and 6 loners (no relatives). The loners weren't sad though.
I had this fortress almost to the stage where the king was to arrive and I went to play another fortress for a while. That one reached 200 Dwarves with almost no extended families, and a lot of loners. It was in a terrible location. A dragon melted everyone in that fortress, so I went back to my wonderful fortress. The King arrived, and lo and behold, three of his sons already lived in the fortress as adults. Between his relatives, his wife's relatives, and their own descendants, they combined three of my 17 clans into one. The total membership of that clan is 73, out of a total of 212 Dwarves. About half of those are adults, mostly married to living spouses.
There is a small flaw in the genealogy function in DF. While Grandparents are displayed, Grandchildren are not. Thus, until you come upon a grandchild of someone you may not notice that someone who has no relatives listed actually is related to another Dwarf in the fortress. I have at least one like that.
I had one interesting case. A Dwarf's and his wife brought the wife's father with them when they migrated. He was listed as married but his wife had not come along. I checked in Legends mode and the missing spouse had become a vampire. I checked the graves of my three vampires, but she was not one of them. The real names of the vampires show on the remains in the coffin, and on the slab if you make one.
Edit:Fixed typo.