All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, a Muslim should not be a curser, a slanderer or obscene or vulgar.
There are many Prophetic narrations from the Prophet which warn a person who curses another person who does not deserve to be cursed, and renders the sin of the curser like the sin of a person who kills a soul without right, among which is the saying of the Prophet : "Cursing a believer is like killing him, and whoever calls a Muslim a disbeliever, then one of them will deserve the title." [At-Tabaraani]
Moreover, the Prophet said: "When a person curses something, the curse ascends to the heavens, but the doors of the heavens will be closed to it, and then it descends to the earth but the doors of the earth will be closed to it, then it goes right and left and if it does not find someone to whom it can be applied, it reverts to the person who was cursed, if he deserves it, then [it stays with him], otherwise it reverts to its sayer."
For more benefit on the prohibition of cursing, please refer to Fataawa 91777 and 86030.
Therefore, it becomes clear from the above evidence that cursing someone who does not deserve to be cursed is forbidden.
As regards the number of sins that one gets by doing so, then this is a matter of the unseen, and it is not acceptable to speak about it without a textual evidence from the Quran or the Sunnah.
Allaah Knows best.