I got this during present day mission 3. There are 19 computers I hack, (I checked) though only 17 counted, I believe some don't count because there mission specific, in any case I've listed below how many CPUs I've hack on each floor.
There are 3 on the 15th (security room, office, and the secretary's, hack hers when see gets called away). 1 in the lobby security room, it's on the right side if your coming from the elevator. There's 15 all over the 2nd floor, 1 of which is in the security room.
These numbers may be different for everyone and if that's the case you should have or be very close to unlocking this achievement by this point in the game.