With Moses and his dealings with God, there were witnesses: two million Jews.
On numbers, see Wikipedia:
Numbers involved in the Exodus
Exodus 12:37 refers to 600,000 adult Hebrew men leaving Egypt with Moses, plus an unspecified but apparently large number of non-Hebrews ("A mixed multitude also went up with them" - Exodus 12:38); allowing for women and children, the total number involved may have been two million or more.[1] Egypt at the time might have supported a total population of around 3-4 million, maybe even up to 6 million,[2] although Napoleon estimated a only 3 million when he invaded in 1798; in any event, the numbers given in Exodus 12:37 seem to represent something between half and almost the entire probable population of Egypt.
The logistics of the Exodus also present problems. A simple calculation shows that a group of 3 million walking 10 abreast with 6 ft between rows would extend for around 340 miles (3,000,000 / 10 * 6 = 1,800,000 ft. = 340 mi). The "very many cattle, both flocks and herds" which accompanied the fleeing Hebrews, plus straggling children and the elderly, would have increased this distance. Recent archaeological research has found no evidence that the Sinai desert ever hosted millions of people, nor of a massive population increase in Canaan, estimated to have had a population of between 50,000 and 100,000, at the end of the march.[3]
Hebrew University professor Abraham Malamat points out that the Bible often refers to 600 and its multiples, as well as 1,000 and its multiples, typologically in order to convey the idea of a large military unit. "The issue of Exodus 12:37 is an interpretive one. The Hebrew word eleph can be translated 'thousand,' but it is also rendered in the Bible as 'clans' and 'military units.' There are thought to have been 20,000 men in the entire Egyptian army at the height of Egypt's empire. And at the battle of Ai in Joshua 7, there was a severe military setback when 36 troops were killed." Therefore if one reads alaphim (plural of eleph) as military units, the number of Hebrew fighting men lay between 5,000 and 6,000. In theory, this would give a total Hebrew population of less than 20,000, something within the range of historical possibility.
On what they saw:
And all the people saw the thunderings, and the lightnings, and the noise of the trumpet, and the mountain smoking: and when the people saw it, they removed, and stood afar off.
And they said unto Moses, Speak thou with us, and we will hear: but let not God speak with us, lest we die.
And Moses said unto the people, Fear not: for God is come to prove you, and that his fear may be before your faces, that ye sin not.
And the people stood afar off, and Moses drew near unto the thick darkness where God was.
And the LORD said unto Moses, Thus thou shalt say unto the children of Israel, Ye have seen that I have talked with you from heaven.
But all they really saw, according to the account, was lightning and smoke.