There is a fabulous discussion with regards to the inconsistencies in the Biblical Exodus by Dr Louay Fatoohi and Prof. Shetha Al Dargazelli in their co-authored book. 
It is quite a lengthy discussion but well worth the read. Dr M. M Al Azami offers an excellent synopsis of the problem our Christian and Jewish brethren encounter with regards to the Biblical numbering of Israelites at the Exodus.
Number of Israelites at the Exodus estimated at 2,000,000 (two million)
"One year after the Exodus, Moses and Aaron counted the total number of men who were at least twenty years old and of fighting strength. Their tally yielded 603,550 Israelites [see Numbers 1:20-46].
The Levi tribe was not included in this figure, and neither were the females of all ages, old men, and any young men under twenty. Taking these groups into account as well, we can infer that – according to the OT – the total number participating in the Exodus probably exceeded two million Jews.
I will leave it to the imaginative reader to surmise how a tribe of seventy people, freshly arriving in Egypt, were able to multiply in excess of two million within a mere 215 years, especially when their male newborns were being systematically killed for the previous decades. Such is the OT which rests in our hands today." 
Summary of the problem
You have the OT asking us to believe 70 new settlers in Egypt yielded a progeny of ~2 million in roughly 200 years. Now, even the most fundamentalist of Christians will start asking questions as it is very unlikely the Biblical account is accurate:
The self-refuted claim here is that the descendents of less than 100 persons counted some 2-3 million after four generations! This contradiction has already been picked up by scholars [e.g. Houtman 1993: 512].
However, in the interest of fairness the fundamentalist Christian answer can be presented and scrutinised…
Fundamentalist Christian answer analysed?
"One way of harmonizing the above contradictions is by suggesting the Hebrew word dor is not to be understood as “generation” in the modern sense. Thus, in his attempt to show his suggestion that four dor equal 400 years is not a mere harmonization exercise, Kenneth Kitchen, Professor of Egyptology at Liverpool University, points out that in Ugaritic and early Assyrian sources, the word daru, and hence the Hebrew word dor, can mean a span of eighty years or more (Kitchen, 1996:54).
Yet even if this was a valid definition of the term dor it does not explain why there were “four” rather than “five generations”, particularly given that 430 is the exact number of years anyway.
This explanatory attempt implies that it was merely fortuitous that Exodus 6:14-26 shows Moses to be from the fourth generation of the descendents of Jacob. Kitchen also eliminates the contradiction caused by Exodus 6:14-26 by suggesting that these verses do not give a full genealogy of Moses, despite the fact that there is no reason, not to give the full genealogy, apart from the harmonization motive, to suggest otherwise." 
An easier answer to the problem – suspect the SCRIBES!
Let’s be realistic. We know scribes were changing the New Testament through inadvertent errors (or at times utter dishonesty). Simple reasoning would lead us to suspect the OT scribes were of a similar nature too – thus producing the problem in Numbers 1:20-46.
It’s a simple answer. Our fundamentalist Christian friends will jump through hoops to circumnavigate such simplicity.
To persuade our fundamentalist friends to consider this reasonable approach (of suspecting the scribes) we can appeal to Jeremiah. Here is Jeremiah to tell us the scribes were unreliable and untrustworthy:
"'How can you say, "We are wise, for we have the law of the LORD," when actually the lying pen of the scribes has handled it falsely? [NIV,Jeremiah 8:8]
Allah knows best.
 History Testifies to the Infallibility of the Quran – Early History of the Children of Israel, Dr Louay Fatoohi and Prof. Shetha Al-Dargazeli, Adam Publishers and Distributers, Delhi, 1999, p 8-18
 The History of the Quranic Text from Revelation to Compilation – A Comparative Study with the Old and New Testament, M.M. Al-Azami, UK Islamic Academy, Leicester, 2003, p216
 History Testifies to the Infallibility of the Quran – Early History of the Children of Israel, Dr Louay Fatoohi and Prof. Shetha Al-Dargazeli, Adam Publishers and Distributers, Delhi, 1999, p 12
Sexism: A reason to the change the Bible?
Numbers and the Bible do collide on more than one occasion