Weapons can be equiped with 2 sockets
Armor can be equiped with 4 sockets.
Different level or type of weapon or armor may require different amounts of socket stones. (never seen one requiring only one socket).
Take the item you wish to add sockets on to the PWI Boutique Agent located in major cities, then you can find out how many socket stones you need for your item.
Socket stones can be purchased from the boutique. For 1 gold you get 15 socket stones.
PWI does put these on discount every now and then.
Players also sell thse in catshops and range in price from 6k-10k each socket.
Once you have the required number of socket stones, return to the PWI Boutique Agent to attach them to your item.