The minimum round # depends on the tier, from 5 to 500.
Until you've reached the saturation, it's not very useful to look at the average because all the bad rounds count in which will be thrown out later.
For the same reason it's of no objective use to abort rounds "to protect the average".
2 9-hole rounds are better than one 18 hole:
- it's easier to get a good score,
- they count double: Each 9-hole round's score is doubled for averaging, both count.