Shulchan Aruch, Orach Chayim 684:3 says (in my own translation):
If rosh chodesh Teves comes out on Shabas, we remove three Torah scrolls, and six people read in the weekly section and one in that of rosh chodesh, starting from "Uv'yom hashabas"; the maftir reads in the third scroll....
Mishna B'rura comments on "[six read] in the weekly section":
Likewise if seven, or more than seven, want to read, that's allowed also; the point is that there are no fewer than seven people between [the first two scrolls].
Shaar Hatziyun adds:
Likewise, five men can read from the weekly section
— meaning, and two from the second scroll. Presumably the same rules would apply to Shabas rosh chodesh Adar and Shabas rosh chodesh Nisan.
My questions are:
- Does anyone know of a synagogue in which only five people read from the first scroll? Why do they do this?
- There are some synagogues that add one aliya every week, so eight (plus the maftir) read. In weeks with three scrolls, do they add the aliya to the first scroll or the second?
- There are some synagogues that add many aliyos every week, stopping as often as every three p'sukim (where allowed). Do they also split the second sefer between two people?