Did the DS9 producers consider adding Andrew Robinson to the main cast?

[Oddish]

I don't think the nine-character limit applied in the Kurtzman era. But in general...

TOS: 3 (with 4-5 semi-regulars)
TNG S1: 9
S2-4: 8
S4 on: 7
DS9 S1-3: 8
S4-7: 9
VOY: 9
ENT: 7

LD S1-2: 8
LD S3-5: 9
SNW: 8 or 9
PRO: 9, includes villains

 

[Cap'n Calhoun]

PRO: 9, includes villains

Warning: Way too much information on this topic ahead.

In addition to the initial nine, season 2 added the following to the opening credits when present:

(Using spoiler tags in case anyone still hasn't gotten around to Prodigy season 2 since it's sorta-recent and not the main topic of this thread. But, seriously, watch Prodigy season 2; it's great!)

Spoiler

Jameela Jamil as Asencia/The Vindicator
Wil Wheaton as Wesley Crusher
Robert Beltran as Captain Chakotay
Robert Picardo as The Doctor

All thirteen were credited for 2.10-2.13. Some are not credited in the opening in every episode where they appear to avoid spoilers.

That said, that's going off the opening titles, and Prodigy has the curious habit of listing all of the opening credits actors again in the closing credits, something no other Trek show (including animated shows) does, though it's common in the films.

For season 2, they listed the six "kids" followed by "and Kate Mulgrew as Janeway" in each episode (detailing which versions of Janeway were present), resulting in consistently having seven "main" listings before the guest stars. For season 1, John Noble and Jimmi Simpson were listed before Mulgrew in the end credits when present (as with the opening credits for both seasons), so season 1 had a peak of nine in the closing credits if you stop counting with "and Kate Mulgrew".

TL;DR: Peak of 9 like you say if going by the closing credits; peak of 13 if you go by the opening credits.

LD S3-5: 9

In terms of opening credits cast, Lower Decks stayed stable at eight for all five seasons. Gabrielle Ruiz as T'Lyn did became a de facto regular for seasons 4-5 if that's who you meant, having appeared in 14/20 episodes, but she was never in the opening credits. (Conversely, Fred Tatasciore stayed in the credits even for the early season 2 episodes where Shaxs was dead.)

 

[CoveTom]

Interestingly, despite the TV Tropes assertation that he "remains as a special guest star", he was only credited as "Special Guest Star" in (I believe) eight out of his 37 appearances... all in season 4, except for one in season 5 ("Things Past") and in the series finale. (And that episode "promoted" several guest stars to "Special Guest Star," including Rosalind Chao and Penny Johnson, who were always credited as regular guest stars until then.)

So if he was looking for a prominent "Special Guest Star" credit, it didn't happen; he was credited as a regular guest star for most of seasons 5-7.

I would be interested to know how they decided who to give "Special Guest Star" billing to in the finale. Marc Alaimo didn't get it. I would think he of all people should; he'd been there since the pilot. Neither did Aron Eisenberg. I'm sure there was a logic to it; I just can't figure what it was.