I voted 10
I voted a 10 for extra smiting.
I thought about the ideal feat progression for a Paladin and came to the following conclusion.
Human Paladin
1: Weapon Focus: Long Sword, Power Attack
3: Divine Might
6: Extra Smiting
9: Improved Critical: Long Sword
12: Power Critical: Long Sword
15: Extra Smiting
18: Extra Smiting
There are some assumptions: I was assuming a campaign where your Paladin doesn't get to fight from horseback all the time. I was also assuming a campaign where you might someday find a holy sword. I was also assuming you are playing with only the WOTC books (non-WOTC books have other feats that are nice).
If you have no chance of ever finding a holy sword, you might pick a weapon other than long sword (or substitute additional extra smitings for the weapon feats). If you plan to usually fight on the back of your mount (a campaign that focuses on tournaments or battlefields), then you will definitely want the mounted feats. If Divine Might requires a standard action to activate, you might drop it and trade it for something else (i.e. if you are not playing with the FAQ interpretation).
If you are going EPIC, extra smiting is incredibly useful, because of the way it multiplies the EPIC Great Smiting feat (the only feat an Epic Paladin will ever want to choose).
Tom