Upon rereading the question, I realize I've answered something slightly different - how to limit the multiple channeling to evil witches, rather than how to limit the strongest spells to evil witches. But if evil witches can pull off multiple channeling when good ones can't, that too limits the strongest spells to evil witches, so I think it still works as an answer.
The channeling process begins with a minuscule flow of power from the witch to the donor, and that flow persists while the channeling is ongoing - the witch's power forms the structure through which the donor's power flows. When the donor's power flows to the witch, their power mixes together and the structure is now made of the mixed energies.
When the witch is channeling multiple people at once, everyone's power mixes together and all links start to contain everyone's power. But normal, non-witch people react adversely to contact with another normal's power. The more people being channeled, the larger the power flow and the longer it lasts, the worse the health effects. Harm to her (probably unwilling) donors isn't something that much concerns the evil witch, but a good one would probably stay away from such practices unless absolutely necessary.