They say that 3-4 strips per page is usually fine but does it have to be or can it be depending on how much room there is on each page?
I think you mean panels per page.
I used to get paid to write comic scripts. The script is basically the comic book. The panel is the box on the page with the picture/dialogue/caption/etc. in it.
There are 1–9 panels on a page. Three rows of three columns is the 9-panel page. I’ve never done a 1-panel page but have seen it done. I typically had less than 9 panels per page because I usually had some larger panels which took up more room. The last panel on a page should be a cliffhanger.
You define the shape and size of each panel on the page for the artist. Basically, the layout of the page. And of course what goes inside each panel. And where the panels are situated is important. There’s a defined way to read the panels — left to right and top down. I don’t remember the specifics.