The maximum junction temperature is usually the same as the maximum storage temp. The upper limit of this is 160 degrees celsius for the MAX6818EAP in the datasheet.
The Theta-jc and Theta-ja for the MAX6818EAP+ package are as follows:
Theta-jc for single layer board = 33 (ºC/W)
Theta-jc for multilayer board = 32 (ºC/W)
Theta-ja for single layer board = 125 (ºC/W)
Theta-ja for multilayer board = 84 (ºC/W)