Captain Steele can have up to three wombs at one time but must have an existing vagina for each womb. You get the womb at the same time you add a vagina to your Captain Steele.
To be precise, the game doesn't track how many wombs you have; rather, everything that a womb would do (mainly getting pregnant) is included with the vagina instead, so the game implicitly assumes that each vagina has one womb attached to it.