Reports, dissertations and bound documents rarely want a number on the cover, and a chapter pulled out of something larger often has to carry on from where the previous part stopped.
Leaving pages bare and choosing the first number are different things, and mixing them up is why numbering so often comes out one off. Leaving pages bare decides which pages get a number at all. The first number decides what the counting starts from.
They work together, which is what makes a section of a longer report straightforward. Skip the cover and start at twenty four, and the page after the cover is numbered twenty four. Both are needed when a document will be bound with others.
One detail that trips up other tools is a landscape page inside a portrait document. A rotated page reports its dimensions the wrong way round, so a number placed without allowing for that lands along the side of the page instead of the bottom. Here it appears where the reader sees the bottom.
Say a report has 20 pages, with a cover and a contents page first. Set Leave the first pages bare to 2 and First number to 1. The third page is then numbered 1, and the last is numbered 18. If you want the numbers to continue from an earlier part, set First number to where it should resume.
In the 3 of 12 formats, the total counts only the numbered pages, so this example reads Page 1 of 18, not of 20. That is deliberate, because the cover is not part of the numbered section. The preview shows the first numbered page, so you can see it before you save.
You can leave any number of pages at the front bare. Everything after that is numbered.
It goes in the margin, and you can move it further from the edge or make it smaller. The preview shows exactly where it lands.
Not from the saved file, because they become part of the page. Keep your original, which is never changed.
The total counts only the pages that are numbered. The two bare pages at the start are not part of it.