For orchestral parts recommended stave sizes are 8.5 or 8mm (the larger size preferred for string parts which often have two players per desk. (Note that A4 paper size is considered a reluctantly-acceptable minimum.) That's an absolute maximum of 10 staves per A4 page. 7mm is OK for piano music. That's 11 or 12 staves on A4. For a study score of a big orchestral work, we see as small as 3.5mm which fills A4 with - lots of staves :-)
If buying pre-printed manuscript paper, it almost certainly wouldn't be A4. On a larger size 12-stave could be considered standard. But I'd make 10-stave the standard if you're stuck with A4.
(One great thing about today's ubiquitous computer music preparation is that we can suit size to content.)