The most commonly used ratio is 1 part fragrance oil to 16 parts melted wax, so 29.6ml of fragrance oil for every pound of solid wax is a safe bet for highly scented candles. This will produce a strong scent when the candle is burnt that most people enjoy. Any more than this and you will have a candle that's too greasy and hard to set.
For a More Subtle Scent
For a less overpowering scented candle, the amount of fragrance oil used can be cut in half, or 29.6ml of fragrance oil per 907gr of melted wax. Those sensitive to perfumes often prefer a subtler scent and may use even less if their personal tastes call for it.
While there is certainly a limit on how much oil you can use before compromising the structure of a candle, there's no set minimum of fragrance oil required, so candle-makers should experiment until they find the perfect ratio. That being said, many prefer the stronger scent of 1 part fragrance to every 16 parts wax, making it the best choice for most.