It's not hard in the U.S. Bitcoins are treated as capitol gains. You need to know:
What you paid for the coins.
What they are worth at the time you sell or spend them.
How much and what percent you will pay varies depending on your bracket and other things. It could be as low as zero but is very unlikely to be more than 20%. You will have to do your taxes to find out, but you will only be paying taxes on the profits and you don't need to say how many coins you own.
If you lost money then you can also claim a loss and reduce your taxes.
Few like to pay taxes, but the good news is that this will be less taxed than the money you work your ass off for.
What if you legitimately forgot/lost said data? as some people has mentioned before, it is more than probable that a lot of your trades were done under exchanges that are now dead and there is no way to recover all the lost data... so in hell are you supposed to try to figure that out?
