I think that the number is between 100,000 and 200,000, though Gimli and Legolas would know for sure. I've found these:
"With 100,000 orcs participating in the massive Battle of Pelennor Fields, Taylor and his crew decided to provide them with distinguishing features. "We played around with prosthetics. We created an Elephant Man-style orc and characters with more mutated growths."" Full article
here .
And there was
this: "The battle pitted 120,000 digital orcs, each with its own AI fighting style and armour against 8,000 digital Riders of Rohan."
And
this that mentions an orc army "200,000 strong" (page 4).