An electron carries one elementary charge of the type we call "negative".
Science cannot explain what constitutes charge, but there is convincing evidence that
only two types of charge exist, which we historically call "positive" and "negative"
charge is one of many quantities that are "quantized", meaning that the quantity exists only in discrete amounts, with a smallest unit that cannot be divided into smaller portions. This is called the elementary charge.
In SI units, the elementary charge is equal to
Electrons carry one unit of negative charge, while protons carry one unit of positive charge. Extremely precise experiments have been conducted an an attempt to determine whether there is a difference in the amount of charge on a proton and that on an electron, and the answer seems to be no difference at all.