Page count isn't a matter of taste, and it isn't a house rule either. You get it by dividing airtime by the minutes a page of that kind needs.
How do you work out the number of slides?
Start from the calendar entry. Subtract the discussion reserve, the time the audience will spend talking instead of listening. For a decision meeting it's larger than most authors believe. What's left is airtime. Divide it by the minutes per page the format demands: about 3 minutes for a page a working group will interrogate, about 2.5 for a page a committee will accept or challenge, about 1.5 for a status page nobody will question.
| audience | slot | discussion reserve | airtime | min/page | pages |
|---|---|---|---|---|---|
| Deal-team working session | 90 | 30 (33.3%) | 60 | 3.0 | 20 |
| Investment committee | 45 | 25 (55.6%) | 20 | 2.5 | 8 |
| Board sub-committee | 60 | 35 (58.3%) | 25 | 2.5 | 10 |
| Weekly team update | 30 | 10 (33.3%) | 20 | 1.5 | 13 |
Which column did your eye go to first? Probably the last one.
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How does the audience change the slide count?
Read the reserve column before the page column, because that's where the audience is described. An investment committee reserves 55.6% of its slot for argument, and a working session reserves 33.3%. Those two numbers say more about what kind of document each wants than any style guide does. A committee doesn't come to be shown things. It comes to interrogate a recommendation it could have read beforehand, so eight pages is what fits. That only feels rude to an author who wanted twenty.
A pre-read has no airtime at all, which changes the calculation entirely. Price it as a reading budget instead. If the reader will give twelve minutes and a page carrying a full dash line takes about forty-five seconds to read properly, the ceiling is sixteen pages. That's generous next to a presented deck, and it explains why the pre-read version of a deck is usually the longer one.
What happens when a deck has too many slides?
The failure mode is worth pricing because it's so common. Take a twenty-four-page deck into a 45-minute investment committee that reserves 25 minutes for discussion. It gets 20 minutes of airtime, which is 50 seconds a page against the 2.5 minutes the format needs. That's a three-fold over-build. Nobody in the room reads it as thoroughness. They see a presenter racing, and the committee stops the presenter on page 4 to ask the question the deck was going to answer on page 19.
So decide the page count before you draw the first card, and cut the cards to it. Decide it afterwards and a deck with a nine-page argument turns into fourteen pages of accurate material that nobody has time to hear.