Moses
The number of Israelites that did not leave Egypt is not mentioned in the Old Testament. It is estimated about 2 million to 3 million did leave Egypt, including men, women and children.
The Israelites went to Egypt because there was a famine in the land of Canaan, where they were living. Joseph, one of the Israelites, had risen to a position of power in Egypt and invited his family to seek refuge there.
Pharaoh heart was hardened , so he would not let the Israelites free.
According to the narrative in the Torah, all of the Israelites left Egypt.
the Israelites did not leave during passover. When they did leave it was Moses who lead them.
They didn't. There are no records of any Israelites being in Egypt apart from the Old Testament. No Egyptian records or any other records mention them at all.
The Israelites left Canaan and fled to Egypt because of famine. The famine was caused by a severe drought that left their crops dead and their people hungry.
The problem presented at the outset of the exodus from Egypt by Israel was their enslavement and oppression by the Egyptians. The Israelites were forced to work as slaves under harsh conditions, and they cried out to God for deliverance.
According to tradition (see Rashi commentary to Exodus ch.13), the majority of the Israelites had become idolaters in Egypt (Ezekiel ch.20) and were not deemed worthy of joining the Exodus - or didn't want to. Those who did leave Egypt were 20% of the Israelite populace (Rashi, Exodus 13).
Famine in Canaan. Much the same as what caused many Oklahomans, Texans, and New Mexicans to leave the plains and travel to California in the 1930s.
The first time he left Egypt, Moses left alone. The 2nd time he left, he was accompanied by a throng of up to several million people, comprised of all of the "Children of Israel" and many Egyptians who chose to leave with them.
The Israelites ate unleavened bread during Passover to commemorate their hasty departure from Egypt when they did not have time to let their bread rise. Eating unleavened bread during this time serves as a reminder of their ancestors' freedom from slavery.