I believe that there are no hard and fast rules for steads and bloodlines.
There are single bloodline steads, there are smallish steads with three or
four major bloodlines. You just need to rationalise the history.
(Of course, if your clan history extends back beyond the dawn, this may not
be useful).
I find it most helpful to think of bloodlines as environmental adaptations
as much as kinship groups. They split and merge according to the fortunes of
the steads and the resources they can exploit as much as through kin
quarrels or family growth. A bloodline is simply an extended family, but it
is also the unit of work. The steads are organised communally and clan
resources are given by the chief into a bloodline's keeping. They are
remarkably fluid in terms of merging, splitting and branching out. While
most largish bloodlines will have a mixture of cottars and carls, rich
cattle husbands and poor sheep herders, part of their herds (and therefore
riches) are communal.