For example, long the futures contracts.
number of futures contracts = 100
multiplier = 5
purchase price = 20
settled price = 30
The profit = (30-20)*100*5=5000? Is it right?
Do we need to also multiply the multipler 5 for calculating the profit?
To calculate the profit in dollars (or euro or yen or pounds or whatever currency), yes.
No matter what currency, the answer should be YES, right? 
Yup.