how much sugar is required to be added to fruit for jam making

Base ratio is equal weights of sugar and fruit.

Using less sugar is possible but requires considering the following points:

• Sugar is a main aspect of preservation. Using less requires adding alternative substances such as sorbic acid to compensate.
• Sugar is required for pectin to bond. This can be compensated with more pectin and/or longer cooking time while ensuring there is enough acidity (another factor for pectin bonding)
• Canning protects the result better than simply jarring.

The sealing determines how difficult it is for germs to get in. The chemicals determine how well it resists to germs once they do get in.

I do not like my jam too sweet. I blend 5 parts (by weight) pineapple to 2 parts sugar, and then cook with cloves and cinnamon (no pectin). I cook this to 104-105C. I refrigerate my jam. The required 'jammyness' of the jam seems to be purely a function of temperature - if I cook my jam too long and the temperature goes higher, then it will be too thick to spread.

There does not seem to be any reason for me to use any more sugar, given that I live in the tropics and I refrigerate my jam (because the heat, humidity, mold, etc. is so intense out of the fridge) and have access to pineapples all year round, so I can just make more when I run low. If you have an annual harvest and are not going to keep your jam in the fridge then maybe your results will vary and you will need more sugar to prevent spoilage.