need help on how to implement case statement in DAX

You can use a Switch statement to achieve this.

When to use Switch Statement instead of IF Statement in PowerBI | MiTutorials - YouTube

Good Morning, Anita, 

You should be able to do this with a Switch(True()) function. 

For a calulated column, your code should look something like this. :

Calculated Column =
SWITCH(
TRUE(),
TableName[Acc Number] = "54112", TableName[Amount],
TableName[Acc Number] in {"54112", "56019", "44291", "442901"}, TableName[Amount],
Blank())

The syntax for a measure changes some, so be aware of that. I used TableName as a placeholder for whatever your table is called in your model.

I wasn't sure what your return value was for anything outside the parameters so I put a blank() to make it easy. 

Hopefully this helps!

Thanks,

JTB