Can my EOS 1200d autofocus with1.4x canon teleconverter and canon 300 mm f4 lens. What will be my maximum aperture using both of these.
According to Canon's website, the
1.4x Extender III is compatible with
EF 300mm f/4L IS USM and
*When Extender EF 1.4x III is attached, the lens maximum aperture will be reduced by 1 stop. Refer to the camera and lens instruction booklets to see if there are any AF compatibility limitations due to the reduced max aperture.
Autofocus will also work given that you have enough light for the camera to be able to focus and you are using the largest aperture possible.
As far as I can see, the camera limits the autofocus to a 5.6 aperture
You will also get a lot more information from this answer: https://photo.stackexchange.com/a/16400/26160