Also, Visit Number is based on ALL Time data... whereas Unique Visitor is based on your report period.
For Visit Number, the first visit could have been recorded years ago.
As @mohammadz said, a user can have multiple visits in a month. or in a day.. each of those visits will have it's own incremental number. So let's say I as a user came to your site 5 times (if it's my first time to your site, I would show as Visit Number 1, 2, 3, 4, and 5 (and potentially visit 2 and 3 could have happened on the same day)... but if I have been using your site for years, those 5 visits could be represented by Visit Number 146, 147, 148, 149 and 150.
@Hyeran I am not sure what you mean by "As I understand, the visit number is counted as a unique visitor, so if I visited the website yesterday and visited today, it is recorded as Visit number 2 and not recorded as visit number 1"... within the same report period, every visit will count with the proper visit number...
Visit Number 1 will not be dropped from that visit once Visit Number 2 occurs.... each visit will maintain its visit number associated to the visit.