No, a silencer (also known as a muffler) will not significantly affect the mileage of a Royal Enfield or any other motorcycle. The primary purpose of a silencer is to reduce the noise produced by the exhaust gases as they exit the engine. It contains baffles and chambers that help to muffle the sound and make it quieter.
The primary factors that affect the mileage of a motorcycle are related to its engine efficiency, weight, aerodynamics, and riding conditions. Installing a silencer alone should not cause a noticeable decrease in mileage. However, if the aftermarket silencer you install affects the back pressure or the overall exhaust system performance, it could potentially have a slight impact on the engine's efficiency and thus, the mileage. But this impact is generally minimal.
It's worth noting that modifying the exhaust system of a motorcycle, including installing an aftermarket silencer, may have legal implications depending on your jurisdiction. It's important to ensure that any modifications you make comply with local laws and regulations.