I am intersed in renting an apartment that is still occupied, the landlord says it will be available in 2 month, he is askoing for a 500 deposit to hold the apartment and not show it to anyone else, is this legal?

Yes, it's legal. It's a contract between you and the landlord. The terms are just a little bit unusual. I have seen this in apartments which are either VERY reasonable and in high demand or in area which are in high demand.

Should the deposit be refundable if I change my mind or in the case that the tenants occupying have not left the apartment on the date I am scheduled to move in?

First your agreement should be in writing and the details about refund or not spelled out. He will likely not want to refund unless he has another renter

No, you will NOT get your deposit back if you change your mind UNLESS the landlord finds another tenant. If he does YOU will be responsible for his charges in finding that tenant.

Put wording in the contract that if the apartment is not ready for your occupancy on a certain date then the landlord breaches the contract - and you get your deposit back.