#AT means Number of Attacks per combat round. Sometimes it is written in long-form as "Number of Attacks", sometimes with the abbreviation "#AT".
The example given is of a creature called a "Farmer". It has Armor Class 7, and is Level 0. It has 6 hit points, attacks once per round (#AT 1), and does 2-8 points of damage (i.e. 2d4) on a successful hit. It is worth 16 experience points to kill. Unstated, but this terrifying creature is likely to be human and possibly works all day with cabbages or cows or the like.
#AT is often left undefined in the books, but there are many pages in core rule books, supplements, and modules where it is explicitly defined as meaning Number of Attacks. For example:
- D&D Basic Set, DM Rulebook, page 2. published 1981.
- AD&D 1st edition, Book of Lairs, page 4. published 1986.
- AD&D 2nd edition, Karameikos, Kingdom of Adventure, for the Mystara campaign, page 51. published 1994.
In Original D&D, both #AT and Number of Attacks are used to mean the same thing in various places, but I can't find any explicit definition saying that they are equivalent. It was probably assumed to be an obvious or even well-known abbreviation for the war-gamers that made up most of the market for such games at the time.
The abbreviation #AT seems to have disappeared from later editions of D&D. I can't find any mention of it in the core books for 3.5, 4e, or 5e or in modules and supplements written for those versions.