Apparently more than a thousand
You can tell this by the length of the Deeping Wall, the fact that there were several ranks of defenders shown on it, there were several ranks of archers in reserve behind the wall, and the
Rohirrim defenders are described as being a few hundred. From the scene the wall is many football
fields in length (lets say 4), from the density of the defenders as shown by the close up of
Gimli,
Legolas &
Aragorn you can see about a defender every yard (actually more but let's say one per yard). 1200
men on the wall, 1200 men behind it, several hundred in the
Hornburg (whose defenders are very dense), the fact that the Rohirrim only number a few hundred leaves 1000-2000 defenders who are
elves. In the scene where they march in I estimated around 1,000 but it was hard to determine since the scene was dark.
Even if we halve this (say the wall is only two football field in length we still have far more than a thousand defenders the leves must account for.
Just as an aside I point out that the movie wall was far higher then the 20 foot wall described in the book though it's thckness seems about right (wide enough for four men to walk abreast).