Imagine that the Earth was showing always the same face towards the Sun (like the Moon/Earth situation).

At the end of the year the Earth would have rotated once (not zero times).

Edit: Imagine now that you knew only that the Earth was rotating around its axis (not parallel to the orbital plane) once per year. Then either we are in the previous case (no day/night cycle) or there are two day/night cycles.

Here's a nice image that explains the difference between a solar day (meaning the time it takes until the sun is at the same azimuth again) and the sidereal day (meaning the time it takes for earth to rotate around it's axis once): https://qph.fs.quoracdn.net/main-qimg-ab0d69361311b4f15b0064...