what is an ideal number of malawi's in a 60ltr tank?

Not 100% sure but from what I have read previously a malawi cichlid needs at least 75 gallons and your tank is roughly 15 gallons.

If my information is correct then 0 would go in your tank. Sorry

Honestly, I'd say none. 60 liters is only about 16 U.S. gallons. That's too small for most Malawi fish.

Without knowing the species of your 5 fish, or exactly what the pet shop person told you, I can't comment on that.

6 months is not a long time for a Malawi cichlid to survive in an aquarium. Most of them have life expectancies of at least 5 to 10 years.

Edit: Read PeeTee's answer. I agree with it.

0 as they cannot be placed in that size of the fish tank , they need at least 55+ gallon tank the bigger fish tank is better for them as they are aggressive fish . in 60 Liter they will die. a lot of fish can survive in small size fish tank for about 3 yrs . im not surprised why they didn't die yet because its only been 6 months not even a year

As your fish begin reaching sexual maturity they will begin killing one another.

BTW, never as a person with fish to sell if there's room for anymore fish in your tank, the answer will always be "sure kid, plenty of room." They sell fish,not good advice.

2.