I have come across with this regulator and it just specifies the max thermal junction. I can just get temperature from the case, how could I calculate the max temperature possible on the case if I don't have the thermal resistance between junction and case?.
Right there, on page 3, is Tjc, junction-to-case thermal resistance (only for the SOIC, however. Not sure why they left that parameter off for the other packages). There's also junction-to-ambient thermal resistance on the data sheet. If you're not using a heat sink, then that's all you need to know. Otherwise, you're going to have to contact the manufacturer.
If you're of an experimental mood, you can use input
or output ESD diodes, or possibly active pin attributes
as a temperature proxy; a little oven time and some
data points for diode Vf@If, and you've got yourself
a regressable temp function. Need to stay away from
imposed high power which could create gradients on
the die and in the package, and respect any latchup pin
current limits if you're testing the part "live".
In still air, the center case top temperature of the package can be expected to be not much below the chip temperature, e.g. 10 - 20 K. We usually monitor it with a thermal imager.