I've heard that there are many ghosts. If they were all reincarnated as humans, they would have other planets to live on, so the Earth wouldn't be overcrowded？

The Earth will not be overcrowded by the reincarnation of ghosts.

According to the records of the Buddhist Association of the Dharma Realm, Master Hsuan Hua's teachings state that Buddhism's cosmology is extremely vast, encompassing the "three thousand great chiliocosms" and countless planets and dimensions. Sentient beings, according to their respective karma, reincarnate in the six realms and different worlds, and are not all crammed onto Earth. Regarding your question, Master Hsuan Hua offered the following core teachings on "the many ghosts, reincarnation, and population issues":

1. The number of those who attain human form is like "a handful of soil," while the number of those who lose human form is like "the soil of the entire earth."

When a disciple asked, "Why has the world's population increased so dramatically?" Venerable Master Hsuan Hua quoted the Buddha's parable, pointing out that although the world's population seems immeasurable, the actual number of beings who have lost their human form and fallen into lower realms far exceeds the human population by countless times. The number of beings in the ghost realm is indeed enormous, but not all ghosts can be immediately reborn as humans; most remain in the ghost realm to suffer retribution, or are reborn as animals, insects, etc.

2. Sexually Degraded Spirit: (This refers to the phenomenon where a person's consciousness (soul) is fragmented and broken due to excessive indulgence in lust or distraction by romantic love, resulting in damage to spirituality and a decline in spiritual level.) A soul can split into multiple lower beings.

Master Hsuan Hua proposed the concept of "spiritual damage and rupture". When a person loses their human form and falls into a lower realm, their soul (true nature) may be fragmented and diluted. For example, a person's soul after death may transform into several kinds of animals, or even split into 84,000 mosquitoes or ants. This explains why the number of lower organisms is so vast, and why they must undergo an extremely long process of karmic purification to reunite, regain wisdom, and be reborn as humans.

3. The Three Thousand Great Thousand Worlds: The universe contains countless worlds for sentient beings to receive their karmic retribution.

In the Buddhist cosmology, Earth is merely a tiny "Southern Jambudvipa" within the "Saha World." Venerable Master Hsuan Hua often spoke of the formation, existence, decay, and emptiness of the universe, and its boundless vastness. In the vast expanse of space, there are countless worlds, planets, and dimensions. Beings are reborn in the same place because of the same karma (shared karma). Therefore, there is enough space in this universe for all beings to be reborn on different planets or in lower realms according to their karma, and they will not all be crammed into Earth.

The five precepts

https://en.wikipedia.org/wiki/Five_precepts

Buddhist Beginner's Guide：Q&A

https://www.cttbusa.org/vajrastrikes/q_and_a_contents.asp.html

What is Buddhadharma?

https://www.drbachinese.org/online_reading/dharma_talks/kaishrlu-1/volume1-ce-28.htm