Magna is right. Depending on what stages, some require more then others. But regardless on which type of game you are playing in main storyline, all the medals require the same hits, like Venom needs 200 hits for a medal, but that's not all. You also need to keep your wingmen alive too. Or else you won't get the medal even if you hit a certain amount.
Each stage needs as many hits as follows:
Aquas Destroy 150
Area 6 Destroy 300
Bolse Destroy 150
CorneriaDestroy 150
Fortuna Destroy 50
Katina Destroy 150
Macbeth Destroy 150
Meteo Destroy 200
Sector X Destroy 150
Sector Y Destroy 150
Sector Z Destroy 100
Solar Destroy 100
Venom Destroy 200
Zones Destroy 250
Titania Destroy 150
~ Answers taken from the original Star Fox 64 Game