How many times can SD card be overwritten and how to find out the actual number of overwrite cycles

Do I know that one of the SD card can write a few times in the future?

Unfortunately no.

Or do I have to know how my SD card is write so far?

The same, no.

Is there such a register can see this information?

No, IMHO there is not any.

I want to know what the future life of the SD card.

Look for Disk Utility on you dash (as an option press Alt+F2 and type palimpsest)

The limit is not an exact count, and applies to single bits. Generally, the logic on the cards tries to distribute the usage evenly over all bits, so you get a lot more usage out of it than it sounds like. For normal daily usage, the card easily works for five to ten years, so you should not have to worry about that at all.

With very heavy write operations (like defragging a lot), you might be able to reduce lifetime to months, but still the card would not stop working suddenly, it would just slowly lose capacity (as the controller stops using 'burned-out' areas). They also have some spare space that is invisible to the user, and will replace lost areas for a while (until it is used up).

So don't worry, it's more like getting old, not like dropping dead.