Depending on how hot you want it - as above. The heat is in the seeds. Quantity suggest is 2 or 3 chilis per cup of oil. Use groundnut / peanut oil as this is quite neutral and will take the flavour of the chilis.
One way to make it is warm the oil, add chopped chilis, heat for 10 mins, allow to cool and add some cayenne pepper and a couple of drops of sesame seed oil. When cold - leave for 12 hrs to infuse - strain into sterilised bottles and seal but add a few whole chilis for effect...
Also try varying for a different look and taste - include garlic cloves - peeled, whole; star anise; black peppercorns...
If you are handing this out to friends, why not make up a little recipe book for them so they know what to use it for - it may not be blindingly obvious to your pals....add some chopped herbs for a dipping oil for breads, blanched veg or seafood; add to stir fries for heat in the recipe; use on toasted ciabatta or as a base for bruschetta etc etc