How to fit a lot of staves in one page?

A3 is a non-American page size. I'm not even sure what it's dimensions are supposed to be. I suggest you put your score in page view and adjust the scaling until it looks right. You get a vague preview while you are adjusting the scaling so if your entire first system is on page 2 and you want it on page 1 with the title, you will be able to see that happen in the preview as you enter new scaling values.

If you feel the systems are too far apart you may also change the max system distance. See https://musescore.org/en/handbook/layout-and-formatting#style-edit-gene…. Your name looks Japanese to me, so you can click where it says English to the right of the text in this link to see it in Japanese if you need to. This is true of almost all Handbook pages. Hideo_T seems to have translated almost everything.