The supply will be clearly stated on a certificate that comes with the coin (e.g. if there is 450 coins minted, you will see 221/450 on your certificate, meaning that your coin is 221st in the emission). Of course you don't have to believe me, but I don't have any interest in spoiling my hobby project.
It just came to my mind - I could have my contracted producer issue some kind of official statement, in which they claim the exact number of coins produced for me. What do you think? Any other ideas?