A full episode can be done by one single person
from art to animation , it's rare since it's a massive work.
But most of the time , the results are not so good in comparaison of the classic industry work , wich is obvious.
For sakuga scenes , yeah a single man do them , sometimes others help him to animate with drawing in-between , sometimes not
I explain bad in my english but if you are interested in this aera
check this
You'll discover , the Nakamura , mitsuo iso works etc
http://www.youtube.com/watch?v=4Pm8AYpOJMw
the full list here
http://www.youtube.com/playlist?list=PLA7B7935290D3805B