Please don't make polls.
This depends so much on the particulars that it's basically impossible to just provide a number. We have performed calculations with over 1,000 atoms, but if you crank up the k-points, use maximum basis set, include d-electron, with a big mesh-cut off, the number decreases, of course.
And, not least: it depends on the RAM on the computer. If you have less than 4 Gb, don't expect too much. If you have more, you need to run a 64-bit edition of ATK to take advantage of it, but naturally this increases the simulation size.
For a general number, that somehow indicates the capabilities, think 500 atoms. But, as mentioned, this number can easily be both 2x and 1/2 depending on the conditions.
