Major portraits are divided between Russian Museum and Tretiakov Gallery.
But there are a lot of the good painture by him which were sent in various local museums.
As for Tretiakov, there are 2 big only Serov room plus the peintings and graphics which are not exhibited.
I have no Catalogue near me, but I could say that there are the following portraits in Tretiakov:
1. Nicholas (about 1900) famouse one
2. Portraits of Grand Dukes ( Pavel Alexandrovich etc.)
3. Portrait of Vera Mamontova ( masterwork by Serov)
4. Portrait of Ermolova ( famouse actresse)
5. Poster of Anna Pavlova
6. Portrait of commercant Morozov etc.
There a lot of Serov in Russian Museum too.
Portraits of Yussupovs, Orlova, Botkina, Hunting Elizabeth are there.