Seen form max V on VIn 7 V, what it limiting? (Wanna 12 8))
To be honest I have no Idea what you are asking. Please clarify!
If you are asking If an Arduino's input can be 12v it would depend on the board. I would guess that most could. I Know that the UNO can take 12v.
Depends on which arduino you are using.
I want to know, can i set VIn on Mega2560 to 12v?
Sure - just need to watch how much current you will be using.
The more you use, the hotter the regulator will get.