How many factors does an experimenter change?

One.

Explanation:

In a scientific experiment, you should only change one factor, or variable, at a time. All the other factors need to stay the same, or constant. If you change more than one factor at a time, you won't be able to tell which factor is the reason for the change in results.

The factor that is purposely changed by the experimenter; the factor the experimenter decides to change ... What did the experimenter change on purpose? The level of the independent variable. The changes that are made in the dependent variable. So One)