How hot can it be?

Just incredibly hot!

No honestly, really really very very, well very hot indeed, unbelievably, unimaginably hot!

It has been calculated that the centre of the sun runs at about 15 million °C.  15,000,000°C

The faster molecules move the hotter they get and at 10¹⁰ °C electrons approach the speed of light and have a greater mass which allows their temperature to continue to rise. 10,000,000,000°C

Max Planck proposed 10³² °C as the maximum and at this temperatures such densities are created that further temperatures would cause each particle to become its own black hole.  100,000,000,000,000,000,000,000,000,000,000°C

String theorists propose a maximum of 10³⁰ °C.  1,000,000,000,000,000,000,000,000,000,000°C

So now you know more, but of course no one has ever measure such a temperature and probably never will.

Told you it was very very hot.