I'm a relatively new player to warframe and am currently building my first new warframe Oberon. However my goal is to get Limbo and I'm. There aren't any free warframe slots given from mastery ranks. You're probably confusing it with loadout slots which are unlocked from every. You only have two Warframe slots, and limited weaponslots. So if you want to keep the Warframes and weapons you accumulate you will have.