The word appears 16 times.
This novel takes place among less educated farm/ranch hands who, at the time are generally racist in any case. The novel just reflects the characters of the time.
"Sure Ya see the stable bucks' a nigger"
"Yeah. Nice fella too. Got a crooked back where a horse kicked him."
Jesus we had fun. They let the nigger come in that night. Litter skinner name of Smitty took after the nigger. Done pretty good, too. The guys wouldn't let him use his feet so the nigger got him. If he coulda used his feet, Smitty says, he woulda killed the nigger. The guys said on account of the nigger's got a crooked back, Smitty can't use his feet"
"Where the hell is that God damn nigger?" George started at his solitaire lay, and then flounced the cards together..."
"Jesus, how that nigger can pitch shoes."
"He's plenty good," said Slim.
I say something, why it's just a nigger sayin' it." Lennie asked," How long you think it'll be before them pups will be old enough to pet?"
"This is just a nigger talkin', an' a busted-back nigger. So it don't mean nothing, see? You couldn't remember it anyways.
Standin; here talkin' to a bunch of bindle stiffs - a nigger an' a dum-dum and a lousy ol' sheep - an likin' it because they ain't nobody else.
She turned on him in scorn. "Listen, Nigger," she said. "You know what I can do to you if you open your trap?"
Crooks stared hopelessly at her, and then he sat down on his bunk and drew into himself.
She closed in on him. "You know what I could do?"
Crooks seemed to grow smaller, and he pressed himself against the wall. "Yes, ma'am."
"Well, you keep your place them, Nigger. I would get you strung up upon a tree so easy it ain't even funny."
"All right, you guys," he said. "The nigger's got a shotgun. You take it, Carlson. When you see 'um, don't give 'im no chance. Shoot for his guts."