Nobody keeps a list, but it would be a fair bet that many thousands of coins are damaged either deliberately of accidentally each year.
As coins are returned to the bank or the Royal Mint, staff weed out damaged or worn coins and recycle them.
No.
There is no recommended chemical to clean defaced currency. Tampering with or attempting to alter currency is illegal in most countries. If you encounter defaced currency, it is best to contact your local bank or financial institution for guidance on how to handle it.
The lower denomination banknotes get a pretty hard time in circulation and wear out very quickly and are easily torn or otherwise defaced. As with many other countries, Australia replaced the lower denomination notes with coins. The Royal Australian Mint expects to get 40 to 50 years from a coin.
Vandals have defaced the pictograph again.
Vandals defaced paintings on display in the Art Gallery.
apparently Australia has the most coins because our coins keep changing
The first exclusively Australian coins were issued for circulation in 1910. Any coins circulated in Australia prior to 1910 were British coins. Occasionally there were coins from other countries suitably restruck for circulation in Australia.
Flush it?
more resources will be used replace defaced money
While Australia remains a Constitutional Monarchy, the reigning king or queen will be on all of Australia's coins as the nominal "Head of State" (Queen of Australia). Currently, and since 1953, Queen Elizabeth II has featured on the obverse of all Australian coins.
You are in Australia - maybe - and have 4 coins of 50 cents each.
Australia issued the following coins in 1986 - 1, 2, 5, 10, 20 and 50 cent coins were only issued in mint packs, not for general circulation. The 1 Dollar coins was the International Year of Peace commemorative and was the only general circulation coin issued in Australia in 1986. There were also a few Non-Circulating Legal Tender (NCLT) coins issued including $10 and $200 commemoratives.