Dual flush toilets usually use 3 and 6 liters of water depending on which way you push the flush handle - 3 liters for a little job and 6 liters for a major event.
Single low-flush toilets use either 4.5 liters or 6 liters.
If the toilet works each time on one flush, they use roughly half the water of a standard toilet. You would then use half as much a month on the toilet. They do not always flush completely on the first flush and the toilet is only a part of the water bill.
In a modern toilet you use 13 litres per flush.
You use anywhere from 1 cup of water to 5 cups of water to flush the toilet. This is per time you flush the toilet.
It takes 1 bucket of water when we flush.
Compared to a 30 year old toilet? An amazing amount (I just installed two such toilets, replacing 30-year-old models). These are dual flush, and in the low flush mode - perfectly adequate for 70% of flushes - they use 1.3 gallons. The old toilets used almost 4 gallons per flush. A saving of 2.7 gallons per flush, times (how many??) flushes per day, times 365: You are easily talking thousands of gallons a year.
I believe it's 3 gallons
Generally, modern toilets are required to be low flow, which is 1.6 gallons (6L) per flush. Older toilets were 3 gpf (11L). Modern toilets vary in the amount of water required to flush them, from 6L to 3L. Some have a dual flush system allowing solids (faeces) to be flushed with a full volume flush, say of 6 or 4.5L, and liquids (urine) to be flushed with a smaller volume flush of about 3L. It is not possible to flush properly a toilet with less water than that with which it was designed to be flushed. Low volume flush toilets have been designed to work efficiently with a smaller volume of water.
There is about four to five gallons wasted if you flush a toilet but I don't believe that it makes a difference if the handle is loose or not. Unless, the toilet is leaking from the handle or somewhere else.