Please help us help you and fill where relevant:
Your LimeSurvey version: Version 5.6.10+230313
Own server or LimeSurvey hosting: Own server
Survey theme/template: Fruity
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I would like to be able to set an equation depending on the amount of answers we already collected from other respondents. Say, for instance, that the variable can be "A", "B" or "C". If we already have collected 100 A, 90 B and 50 C, I would like to set it as "C", as we haven't collected enough results here. I can't use randomization group, because I have other conditions to apply (not everybody can be A, B or C, it depends on their previous answers).

Is this doable ? Thank you in advance for your answers !

Yes, it is.
But you ignored the questions at the beginning.
Therefore it is impossible to say if you have to install and use a plugin or you can use a different plugin that is already included.

Joffm 

Oh accept all my apologies, I tried to copy/paste the version number from the admin section, but it seems it was ignored as I posted the question and I didn't see it ! I finally typed it directly as I edited my original post, but for reference here it is again: version 5.6.10+230313.

Hi,
why copy/paste? Just write it. You see, in the GUi it is a link. But we inly want to know the version number.

Well,
in this casde you can use the included plugin "statFunctions"
 
Maybe you have to "Scan Files" and activate it.

As you see, there are two function "statCount" and "statCountIF"
[url] manual.limesurvey.org/StatFunctions [/url]
They return the number of already submitted values.

Joffm
 

This is great and exactly what I needed, thank you ! I needed to use "Scan files" indeed. Thanks again !