It has been announced that SpaceX Starship SuperHeavy will need 16 launches with the tankers to refuel Lunar Starship.
16 refueling launches is an extremely high number. According to NASA's own analysis in the updated Artemis III Concept of Operations, only four tanker launches are planned.
Elon Musk was talking about 8 launches, maybe even as few as 4, depending on the exact mission profile and the actual capabilities of the final vehicles.
The highest number that I could find is 14 tanker launches. This comes from the GAO report on NASA's selection process after Blue Origin's protest. Note that even Blue Origin, SpaceX's main competitor who have an incentive to make SpaceX's proposal look as bad, complex, risky, and expensive as possible has only claimed 14 tanker launches.
Why is the Starship going to be empty after reaching orbit
Because it burns the fuel to get to orbit.
If it had fuel leftover after getting to orbit that means that you could instead remove that fuel and add more cargo. In other words: if the Starship weren't out of fuel after reaching orbit, it would be wasting cargo capacity.
In other words, isn't the booster powerful enough to carry a fully loaded Starship into Space?
It is. It can either carry a fully loaded Starship full of fuel but very little cargo, or it can carry a fully loaded Starship full of cargo but very little fuel.
Both of those are useless on their own. The best combination is to carry both Starships full of cargo and Starships full of fuel and let the ones full of fuel refuel the ones full of cargo.
It's simple: every single kilogram you carry to orbit can only be one of three things: structure of the rocket itself (tanks, engines, piping, electronics, antennas, etc.), consumables (fuel, oxidizer, pressurant), or useful cargo. If you want more of one, you have to carry less of something else, and you cannot really choose to remove stuff from the rocket, so you can only decide between consumables and cargo.
why will the tankers have fuel whereas the Starships to reach moon or mars will not
Because the Lunar Starship which goes to the Moon has 100 tons of stuff in it that needs to go to the Moon. The Tanker Starship has 100 tons of fuel in it that goes to the Lunar Starship.