During the Battle of Endor, Emperor Palpatine reveals some information to Luke about the strength of the Imperial forces guarding the shield generator that is protecting the Death Star:
Your friends, up there on the sanctuary moon, are walking into a trap, as is your Rebel fleet. It was I who allowed the Alliance to know the location of the shield generator. It is quite safe from your pitiful little band. An entire legion of my best troops awaits them. Oh, I'm afraid the deflector shield will be quite operational when your friends arrive.
We are not told the exact number of troops comprising the legion protecting the shield generator, but according to Wookieepedia an Imperial legion consisted of 8,192 troops. Even without Wookieepedia, the audience could reasonably infer that an Imperial legion numbered in the thousands of troops (perhaps the size of the Roman unit from which we get the word legion, about 5,400).
With the unexpected help of the Ewoks the small Rebel force was told of a nearby secret entrance to the shield generator's command bunker and so avoided the main guard force. In the ensuing battle a force of about 100 stormtroopers and several AT-STs (an estimate based on the numbers seen during the Rebels' brief capture) fought the Rebels and the Ewoks.
The stormtroopers eventually lost the battle and the shield generator was destroyed, but why didn't they call for reinforcements? If there were about 10,000 troops in a legion and only about 100 fighting the Rebels and Ewoks then only about 1% of the Imperial force was engaged in battle. Even if the stormtroopers were concerned that the attack on the secret entrance was merely a diversion, why not send in more than a mere 1% of the available troops (especially when they started losing)?