How many constellations are there in the sky? Why can't I see some constellations sometimes?

A long time ago, astronomers divided the sky into many areas with imaginary lines and gave them corresponding names. At present, the whole sky is divided into 88 regions, also known as constellations. The darkest star visible to the naked eye is called six stars. There are about 6000 stars brighter than six stars in the whole sky, but no matter how clear the sky is, we can only see half of the sky, about 3000 stars. In fact, near the horizon, dark stars are invisible because the atmosphere can reduce light. Even in a very clear night sky, we can only see about 2000 stars at a time, which is really too few. But this is only with the naked eye. If you use a telescope, the dark star can also see. Judging from the number of stars, six stars are about three times more than five stars and seven stars are three times more than six stars. In this way, the number of stars in each dark level will increase by about three times. Even a small telescope with a diameter of 5 cm can see even a star with a level of 10, because its light gathering ability is 50 times that of the naked eye. There are more than 300,000 stars brighter than 10 in the whole sky. Add the aperture of the lens and gather a lot of light, and you can see very dim stars. If you use a five-meter telescope on Mount Barroma in the United States, you can see 21 stars, including 23. However, the stars are more than that. No matter how big a telescope we use, all we can see are the stars near the Milky Way and the solar system. The Milky Way is thought to be made up of about 200 billion stars. In space, there are about 1000 billion small universes like the Milky Way, so the number of all planets in the universe is 200 billion times of 1000 billion. So, the real number of stars is already tens of billions! !