I have trimmed down to 15 Level 5 countries for better management and protection.
Below 9 countries would be better for newer players as you will not have Rebel attacks on factories to deal with all the time.
These attacks start when an empire has 9 or more countries and can become great annoyances.
If I were to try to reach Level 6 or better I would have to trim down even more.
I would suggest never go above what you can protect.
To have more is more to lose.