I want to know this omnidirectional antenna's maximum distance:
- gain 15 dBi
- frequency 2400 GHz
- power 50 watts
Antennas don't have ranges. The maximum range of a link is determined by many factors in addition to the information you have about the antenna, among them:
The only information you need about the antenna is its gain in the relevant direction (an “omnidirectional” antenna necessarily has less gain than a directional one).
Once you have gathered enough information, you can do a link budget calculation to model the expected performance. Have a look at the answers to this question: What is a link budget, and how do I make one?