Immigrant visas are numerically limited by category and by country of chargeability (which is, in most cases, one's country of birth). What follows is the State Department Visa Bulletin showing backlogs in each immigrant visa preference category for the month of March 2018.
Click the image below to enlarge.
Persons born in countries other than India, China, Mexico and the Philippines should look in the "Worldwide" current on the left side of the page to determine their "priority date". A priority date is established by the submission of a relative visa petition in the family categories, and by either the submission of an application for alien labor certification or by the submission of an employment-based visa petition in the employment categories.
The word "Current" indicates that no backlog presently exists in a particular category. Alternately, the word "Unavailable" indicates that it is not possible to apply for permanent residence in that category.
The dates in the Visa Bulletin can be misleading. Some dates, particularly in the Employment Based Categories, are "current" now, but may backlog before you can say "I-485″. Others, particularly in the Family Based Categories, look closer on the Bulletin than they are in reality. To see the rate of advance in a particular category, check the State Department's Visa Bulletin Archive. This allows you to see Visa Bulletins from February 1995 to the present.
Upcoming month's visa bulletin: April 2018
Archived visa bulletins: February 2018 and before