we have a limit do unlock LFS?
When i pass this limit, what's happen?

Then you are not able to unlock a LFS installation on a fresh computer / Windows installation anymore. Until every Friday of the month. On every Friday this counter gets resetted to 2 again.

It is to prevent LFS installation/account sharing with a friend (I guess).

It happens rarely that you are without unlocks during the week, but if it occurs you can write an email to the developers for extra unlocks. The response to such a request is mostly quite fast.

Quote from cargame.nl :

On every Friday this counter gets resetted to 2 again.

Not entirely correct - every friday you get an additional 1 unlock until the count reaches 2.
Also worth to mention that you get initially 3 unlocks at a purchase of licence and the amount of unlocks is reset to 3 when a new stable version of LFS comes out.