According to Tibia Manual there is no limit to change your character sex.
Extra Services: Character Sex ChangePlayers can change their characters' sex for a fee in the Store. To do so, log on the character that should change the sex. Click on "Extra Services" and select "Sex Change" and then on "Buy Now". Confirm that you really would like to change the sex of the current character and your character's sex will be changed instantly.Note: Your character will wear the default outfit once you have changed its sex. All outfits that you have purchased or earned by completing quests will of course still be available after you changed your character's sex.
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