It really depends on how much strain you can take. If you feel able and strong enough to call another spirit, go for it. I work with at least 2 or 3 spirits in a working. I would however, keep to the recommendation to keep it to "like minded" entities. You can't get anything done if everyone is bickering.
Push yourself (SAFELY!) to learn your limitations and grow. You will know when enough is enough. The challenge is saying enough and meaning it.
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