I saw once that there are about 6 millions of unspent outputs
Is it true? How can I check that?
At block #239314 there are 6187163 unspent outputs (plus 2 that had been abandoned because of non-unique TXID)
There is an RPC command in the official client that can print it, after awhile.
I don't remember the exact word, but I'm almost sure that is starts with "get" and does not take any parameters - see "help" and you should find it easily.