Why there are more stars in the sky some nights than the others?

There are not more stars in the sky some nights than the others, but it is a question of what we see. What really happens is that the state of the atmosphere differs very much at different times, quite apart from the presence of the actual clouds. Even when there are no clouds anywhere, and all over the sky the brighter stars can be seen, the state of the air may be such – whether owing to the presence of a lot of dust high up in it, or for other reasons – that the less bright stars can not be seen. The temperature and the pressure of the air have their own effects in this respect.
Much of the recent advance in astronomy has been due to the fact that great new observations, containing the finest telescopes in the world, have been specially built on the tops of mountains, or at any rate, as high up as possible in parts of the world specially chosen for the clearness of the air. And the higher the telescope, of course, the lesser the amount of even clear air that the light from the stars has to pass through before it reaches the eye or the lens of the telescope.
Additional reading:
Star (Wikipedia)

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