It depends on what encoding you are using. If you look at the database structure of wp_posts, you will see that
post_name field is
So, at most it can store 200 bytes of data. If you are using
UTF-16 encoding, it needs 4 bytes to store an Arabic character. So, at most it can store 50 Arabic characters.
For different encoding, it will be different. See https://stackoverflow.com/questions/4322191/how-many-bytes-do-we-need-to-store-an-arabic-character