For Stronger Scented Candles
The most commonly used ratio is 1 part fragrance oil to 16 parts melted wax, so 1 ounce 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 burned 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 1 ounce of fragrance oil per 32 ounces 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.