for every year that you are in, gannon will sell you equipment that has up to +2 wonderful slots for that number. so for year 1, he will sell either a 1 slotted, a 2 slotted, or a 3 slotted. for year 2, he will sell a 2 slotted, a 3 slotted, or a 4 slotted, and so on.
he sells the different numbers randomly each day. so if you are in year one and checked today and he's only selling a 1 slotted one, check tomorrow, he may have a 3 slotted one.
The maximum number of slots a tool can have -without- indigo wonderful stones is 10 (available from Gannon/Goran starting year 8). The maximum number of slots a tool can have -including- indigo wonderful stones is 35 (but 5 of those slots will be occupied by indigo stones, leaving 30 empty slots).