There were a few, but not very many.
The closest thing we get to a number is here:
Altogether there were about two dozen standing on the wide grassy floor of the dingle, and as many more were marching in.
The Two Towers - Book Three: Chapter 4, Treebeard
This would suggest about 48. There is no further mention of concrete numbers after this point.
There is a mention of the variety of Ents, suggesting there were quite a few, however around 50 seems to cover the variation:
At first Merry and Pippin were struck chiefly by the variety that they saw: the many shapes, and colours, the differences in girth, and height, and length of leg and arm; and in the number of toes and fingers.
Pippin then goes on to list another 8 or so types of trees that they could've been.
A few seemed more or less related to Treebeard, and reminded them of beech-trees or oaks. But there were other kinds. Some recalled the chestnut: brown-skinned Ents with large splayfingered hands, and short thick legs. Some recalled the ash: tall straight grey Ents with many-fingered hands and long legs; some the fir (the tallest Ents), and others the birch, the rowan, and the linden.
The first mention of numbers, suggests something in the 20s, but then again that's only as Treebeard walks in:
Several Ents had already arrived. More were coming in down the other paths, and some were now following Treebeard.
The initial description already lists 4 types of tree at various different "ages":
...some as different as one tree-kind from another, as birch from beech, oak from fir. There were a few older Ents, bearded and gnarled like hale but ancient trees (though none looked as ancient as Treebeard); and there were tall strong Ents, clean-limbed and smooth-skinned like forest-trees in their prime;
If we look at Pippin's descriptions, he suggests that a few seemed related to Treebeard, suggesting around 3 to 5 Ents. Now extrapolating that for the other types, the chestnut, ash and fir all got some form of description, possibly suggesting enough were there to generalise (5-8) and the final 3 had enough to distinguish, which we could assume to be around 3-5 again. Taking the upper and lower bounds, we get about 27 as the minimum up to about 44.
In conclusion, there were definitely more than there were in the movies, but even then there were only around 50 or so Ents, however no specific number is given.