There are a few options you could do here:
Why not type it?
All the user needs to do is select the field and then type the number (with the majority of selections having just 1 number in anyway).
Entering numeric values on a number pad is generally more efficient for users.
There are only 12 options in a list which is probably too long for a drop down (on mobile especially) and given the characters they could be rather tricky to select, typing will be better.
You could use something like this:
...and cap it at 12, or put a supporting note to tell users it caps at 12 (if needed), or simply put a 1 and 12 either side of the plus and minus to show both ends of the scale.
OPTION 3 (which you stated)
A seek bar or sliders (scroll or bar based):
Personally I prefer the first two as they are more efficient and user friendly where as the latter has more room for error (particularly for users with large fingers or any accessibility issues).