According to Stine himself, he didn't use a ghostwriter for the bulk of the Goosebumps novels.
Stine has written 250 books, and he is contracted to write one Fear
Street romance and one Goosebumps story a month, and a Ghost of Fear
Street every other month for the next three years. Largely because of
Stine’s popularity, Scholastic Incorporated experienced a 60 percent
jump in sales from 1993 to 1994, making Scholastic the fifth largest
children’s publisher. As Stine himself proudly admits, "I’m a machine.
I’m cut out for this. I’m usually at the computer by nine in the
morning. Generally I say that I’m going to write twenty pages. I don’t
finish until I’ve done my twenty pages—and that’s how I get so many
books done." Although Stine spends two to three days outlining his
books, once that task is finished he claims, "It takes me ten days to
write a Fear Street book, eight days for a Goosebumps." Stine works
without an assistant or a "ghost writer," but his wife Jane Waldhom
does act as editor.
He also confirmed this in a 2013 Reddit AMA:
Q. I rather enjoyed Goosebumps as a kid. The Monster Blood books, specifically. Did you write all of the GooseBumps books yourself, or
like K.A. Applegate, act as more of a supervisor to other ghost
RL__Stine: I wrote all the GB books myself, believe it or not. Sometimes I had writers help me with the outlines. But all books were by me.
Of course, this doesn't take into account that he might just be lying.