Is there a Hadith about someone who going around the ka'ba 50 times? What is the merit of it?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Ibn ‘Abbas narrated that the Prophet said: “He who circumambulates around the Ka'ba fifty times will have his sins forgiven like the day his mother gave birth to him (i.e. having no sins)”. [At-Tirmithi] At-Tirmithi asked Imam Al-Bukhari about the above mentioned Hadeeth and he said: “It is related by Ibn ‘Abbas”. However, some scholars criticized the narrators of this Hadeeth.

In any case, if someone acts according to it, it is hoped that he will get a great reward as there is no scholar who ruled that this Hadeeth is weak or that it is very weak. This issue is about good [excellent] deeds and some scholars stated that one may act according to a weak Hadeeth if it is not very weak and it is classified under a principle.

Enjoining Tawaaf and urging it and circumambulating as much as possible is something on which there is much evidence.
Nonetheless, a person who acts according to this Hadeeth should meet the following two conditions:

1- One should not believe in it that it is confirmed from the Prophet .

2- One should not continuously practice it over acting on other acts on which the reward is confirmed with authentic evidence.

Allah knows best.