There's a big question as to whether there ever were Israelites (or more accurately, proto-Israelites) in Egypt, as there is scant evidence for the whole narrative either in Egypt or going into Canaan, which has led many people to think that the story is either completely made up, or only has some basic historical elements (and arguments about what those elements are).
Ignoring that issue, textually the Israelites came out of Egypt with a 'mixed multitude' (ereb rab) in Exodus 12:38. Elsewhere in the Old Testament, this refers to 'foreigners' or 'aliens' (Nehemiah 13:3), so it's non-Hebrews came out of Egypt with the Israelites. These can be split into further divisions:
Slaves. There were other slaves in Egypt of different origins, and these attached themselves to the Hebrews on their way out. Some have suggested that the multitude might be exclusively Egyptian, as echoed in the Midrash Rabbah. Manetho, writing in the 3rd century BC, said 80,000 Egyptian lepers and diseased people allied with the Israelites, but this is of course, some 1000 years after the event was supposed to have taken place.
Mixed marriages. Leviticus 24:10 has an argument between two Israelites, one who had an Egyptian father, indicating that some of the people who came out of Egypt were Egyptian or of partial Egyptian descent. Later texts in the Old Testament use the same linguistic root to talk about people who intermarried with other groups.
Mercenaries. The Israelites went out of Egypt equipped for battle (Exodus 13:18), but the word for equipped literally means 'girded around the 5th rib' and elsewhere is used to refer to mercenaries (Jer 25:20, Ezekiel 30:5) so it's possible that in this particular instance, the Israelites as slaves, were not ready for battle (Exodus 13:17), but their mercenaries, a result of mixed marriages, were.
Post-Exodus, the mixed multitude is a pejorative term who appear to be separate from the Israelites but yet at the same time are present. In Deuteronomy 29:10, Moses mentions the 'strangers' who take part in the covenant, and Joshua has those same strangers witnessing the recitation of the blessings and curses.
How many were there?
Again, textually approximately 600,000 Israelite men over the age of 20 came out of Egypt, and that number excludes women and children, but at no point are the 'mixed multitude' given a number, only the Israelites. Numbers are routinely exaggerated in the Ancient Near East, so this could be an example of exaggeration, yet at the same time in the text, the numbers of the Israelites were supposed to be numerous.
To what extent were they Egyptian.
I'm not sure whether you mean the Israelites, or the, er, Egyptians. Moses is quite possibly an Egyptian name (or part of one), and there has been some argument over the years as to whether there are some Egyptian personal names in the group that came out - Moses, Assir, Pashhur, Hophni, Phineas, Merari, and quite a number of other names that pop up in the Pentateuch can be argued to be of Egyptian origin (Alan Gardiner considers 'Mary' (via Miriam) to a name that has survived directly from ancient Egyptian). You should be aware that the academic counter-argument is that the Israelites were never in Egypt, because they're actually Canaanites. This might explain some of the 'Egyptian'-ness of the narrative, as Egypt controlled Canaan (with varying degrees) until about the 21st dynasty (about the 11th century BC), and that might explain the names as well.