You get 2 character slots for completely free accounts. Four for silver. 7 for gold. If you want any more than what you get as the base for the membership you choose you have to buy them. I for example have had the old style all access sub that had 12 character slots for many years. When I dropped it for a regular gold sub I had to purchase the 5 slots for the characters that got locked when I changed sub level. The amount of character slots you have is always going to be base for sub + number of slots you've purchased.
