I was wondering if there was a common thing to do with over the top amount of successes, IE 6 Success and 0 Failures or 8 Success and 2 Failures or whatever combo. Most Cards end at 3 Hammers and I am pretty sure Hammers cannot activate multiple lines like boons can. So what can the extra ones be for?

By the rules: Nothing.

However, that doesn't stop you from giving a descripiton of how fantastically successfull the characters is at what he attempts. Or even adding something extra.

For example: The character is attempting to persuade a NPC. He makes such a good impression he gets an extra fortune dice the next time he attempts a social check versus that NPC.

There's an optional rule in the Game Master's Toolkit allowing you to spend more successes on an action.

There are also a few talents and career abilities that can convert those extra successes into boons or other effects if you look.

we lately thought about making item cards for the player's weapons which might be used for something like that. so if you have for example 2 successes left after resolving the action you might add another damage point to that action...

Part of the WFRP concept is that excessive successes is not "good". You want a mix of successes and boons to make an exceptional roll. Just like too many boons can be wasted, too many successes can be wasted as well.

That said, as was pointed out there are some optional rules that allow extra successes to be used. I myself have used that 2 extra successes on an attack can add +1 damage to the attack, for example.

Games Master Toolkit, pg 39. "Higher Lethality"

Using this alternate method, extra successes rolled for an action that inflicts damage have the potential to trigger additional damage. If more successes are generated than necessary to trigger the max success line of an action, the action inflicts +1 damage per success, up to a maximum amount equal to the level of training the character has in the related action's skill.

Likewise the GM may use the same approach to adjudicate extra successes generated on a non-combat or non-damaging check, by allowing an increased effect based on the total number of ranks the character has trained in the action's associated skill.