The size of Egypt's Christian population is disputed, and the dispute is political rather than statistical.
Egyptian government census figures have not reported more than about four million Coptic Orthodox Christians. The church itself has claimed figures in the region of fifteen million. Press and reference works generally settle on ten per cent of the Egyptian population, roughly ten million, which is a convention rather than a finding, and independent estimates have ranged from under five million to something over seven.
The gap is too large to be explained by definition alone, though definition explains part of it: whether one counts baptised infants, practising adults, or everyone of Coptic descent produces very different answers. The rest is a question of trust. The church holds that Christians are systematically undercounted, and points to a state that has controlled census publication closely and to a legal and social environment in which conversion to Islam is registered and conversion from it effectively is not. The state has not published the underlying data in a form that would allow the claim to be tested.
This is the ordinary condition of minority religious statistics under a government with an interest in the answer, and it is not peculiar to Egypt.
What can be said without a number: the Coptic Orthodox Church is by a wide margin the largest Christian body in the Middle East, Christians are a substantial and long-established minority in Egypt rather than a remnant, and emigration since 2011 has moved a significant Coptic population to North America, Australia and Europe, where the church now has dioceses and bishops of its own.