Supply from mining is always constant.
Wrong. A drastic reduction in hashing power on the network causes significantly less blocks to be found until the difficulty is reset. The next difficulty reset is currently expected to happen on November 1st. Dot-bit.org currently says the last 10 blocks were found at 75% of the normal rate. Thus at the current pace, bitcoins are being added to the market at about 3/4 of the normal mining supply.
supply is always the same no matter how many miner there are. their is always 7200BTC generated pre day .
Wrong. See above.
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