How can i get extra keys for this ?

Yes, you can get extra 3 keys or 2 keys or 5 keys package for optional, Search on Amazon: Vmaisi Magnet Keys

Does this scratch your cabinets?

The magnet it pretty powerful and make contact by hovering over the lock on the inside. Rubbing would scratch your white finish but rubbing is not required to use these stronger vmaisi brand magnets.

Is it easy to remove? Will it stick the painting of the furniture?

Yes they remove clean with alcohol. You need to be careful taking them off, to not peel off any part of your cabinet.

Some locks are not staying in the lock position. Is this a defect?

It turns out the the metal cabinet knob or possibly the metal screw holding the knob had become magnetized. So just moved the lock about a half an inch further away from the metal knob and then it worked!

Drawers of my dresser does not even close properly. How to solve the issue?

For inset drawers, the included tool aids in placing the catch deep enough to allow the drawer to close. Most of the dresser drawers required to mount the lock sideways (it still functions!). The included inset ruler tool works well to solve this.