Take the 2-minute tour ×  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:    1. Does anyone know of a synagogue in which
only five people read from the first scroll? Why do they do
this?   2. There are some synagogues that add one aliya every
week, so eight (plus the maftir) read. In weeks with three
scrolls, do they add