How many pages do you think a resume should be? Is there a deal-breaker number in your head?

I think one page is more than enough across any level. If it can’t be summarised then there is something wrong.
There are links to social media platforms like LinkedIn that could be used to point them for more details.

Depends on the level of experience.

A CxO with 20+ years can have a 4-page resume.

An entry level resume can fit on a single page.

But 2-3 pages is the sweet spot for most people.  

Resumes are now crafted to "match" the keywords from the job ad - in case the company uses some sort of prefiltering A.I. engine. It's becoming harder and harder to find young people willing to learn (beginning of their career), mid-level career individuals with hands-on experience, and executives with track records. I would highly recommend networking-based recommendations and a clear track record.

Industry Standard is One Page (inexperienced candidates), Two Pages (for the majority of the market / candidates), and Three Pages (for deeply experienced and/or technical candidates).

For most resumes, 2 is enough. For a well-experienced IT pro, 3 is the max.