Just google the name of the battle and the first entry should be a wiki, click on it and it shoud say the numbers somewhere! For example I googled battle of the hornburg and got this:
Quote:
Strength
In the book, The Lord of the Rings, The Two Towers, the size of the initial garrison at Helm's Deep for Rohan was nearly 1,000, but many more were coming into the fort from across Rohan. The estimated number of total Rohirrim defenders is 2,000[1] by the time Saruman's army arrived. Merry says that the force that left Isengard numbered 10,000 at least, most marching towards Helm's Deep and others heading off to the Fords of Isen. This number is later qualified by Gandalf: "I have about ten thousand Orcs to manage.",[2] so considerably more than 10,000 when the Men of Dunland are added. Though the battle appears severely lopsided, as Uruk-hai were much better in battle than simple orcs, the defenders managed to hold the fort until a force of nearly 1,000 men on foot[3] led by Gandalf along with a forest of Huorns arrive at dawn in the rear of the hosts of Isengard and surround the Uruks.
Simples!