What do the figures 1, 2 and 6 mean? I saw them pressed on the cuffs after crimping.

Most likely your question deals with die 01 made by KBT. This die has three crimping profiles that are indicated with yellow, red and blue color. After crimping you can see an indented number: 1, 2 or 6, which corresponds to color indication also. This feature was added by KBT as an additional way of control of a correct choice of crimping profile for the particular size of the terminal. 1 for cross-section range from 0.5-1.5 mm², 2 for cross-section range 1.5-2.5 mm² and 6 for 4.0-6.0 mm².
For example, if you notice number 2 on a red cuff you definitely know that there was a mistake in the choice of crimping profile and crimping result cannot be reliable.