Dunno what to tell you about not enough copies.
But as to the exp, yes its 400 exp for 4 players, and 600 exp for 6 players. Adventures are written for 5 players normally.
To scale them up, usually you add 1 more of the lowest level standard creature. And the opposite for the scaling them down.
Some use too few creatures though to do this with...Spoiler:
There's one upcoming with just 2 oozes, with that one your best bet is just to add some more hit points to both the oozes to scale up, or remove some hit points to scale down.