There is no maximum temperature, because there is no limit to the amount of energy you can put into anything.
Absolute zero, minus 273.15 degrees Centigrade, minus 459.67 on the Fahrenheit scale, is theoretically the temperature at which all molecular activity ceases.
It represents an absence of energy.
There is the concept of a lowest temperature because any given body has a lowest energy state, when all possible energy has been extracted.
You can also talk about a maximum temperature for any given body, because at some point it will be hot enough to break up, melt or disassociate, so that at that point it would not be the same body.
However, since there will always be some object or substance that will still exist, the concept of temperature does not have an upper limit, because more and more energy could still be added.
A cosmologist might say that this question is like asking “What is the shortest time?”
Things have been cooling since the big bang, so a millionth of a second later, approximately as far back as our physics will take us now, would theoretically be the hottest known temperature, but a billionth of a second after the big bang would be still hotter.
