There are many ways you, the writer, can go about changing this. The way I would personally go about it is to have the energy input to output ratio become heavily skewed. Let's say under normal circumstances the user inputs 10% of their energy to activate the runes which can then siphon up to 50% of the target's energy which ends up coming out to a 40% max energy surplus to fit with how much energy you want them to siphon. The exact numbers are irrelevant for now and can be changed to your liking.
Now, when you suddenly start activating multiple runes on multiple targets, not only are you inputting 10% of your own energy to each person, meaning a theoretical maximum of ten targets at once which may even result in death depending on how literal you take the energy drain idea but the actual practical result is that due to the strain and complexity involved when activating multiple runes at once, each individual link is weaker or not as highly optimised leading to a reduction in power efficiency. In the end, for two people it requires 25% energy input on your end for each rune to be activated, to gain 50% from them each so in total, your energy surplus is only 25% instead of 40% from each person resulting in a total net gain of 50% energy.
Compared to the one person method, you have now gained an extra 10% of energy output for you to use but at a vastly reduced level of efficiency and this is the optimal circumstance. 227
Just as many may not be capable of draining 40% from an individual depending on their skill or power level, with each witch having varying degrees of efficiency, creating new runes and implementing new ideas to try and improve upon the input/output ratio, many who try multi-link runes end up expending more energy than they get out.
In the example above, even the best when drawing power from two people will only see a 10% increase in power, allowing for some cool moments in your story where they can gain extra power to pull off an ability but without it becoming entirely broken within the narrative you wish to tell.
Then when you go to three people, you may be inputting 40% energy to activate each rune which either can't be done as you go over 100% of your own energy and possibly die or become injured or in cases where it can be done by perhaps activating one rune and draining energy from them which is then used towards draining energy from another three, even then, you end up with 30% output from those three people for you to use which is actually worse than if you had just siphoned one person to the max.
In Conclusion: The more people you choose to siphon from simultaneously, the more energy you need to begin with meaning you must first drain others individually to accumulate the power necessary making it long winded and time consuming but you also increase the complexity and introduce efficiency issues which makes it generally unviable. The exact percentages are of course entirely adaptable to suit what you feel is right within the story, possible allowing up to four or five to be siphoned before you end up with a net loss of energy. That's just up to you.
The only final thing I would add, is that remember, if the witches can drain life from humans and they restrict themselves to drawing that 40% out of an ethical or moral code, they are most likely able to draw life from trees, plants and animals too. In which case, depending on the witches in your story and how they view these things, those codes of ethics may not apply. However perhaps this is not possible within your system and energy can for whatever reason, only be acquired through draining people. 227