Hi,
please, ask again, when you have a finalized workflow and a well defined algorithm which brands have to be displayed.
Let's say, in Q13a there are 4 physical brands (P1,P2,P3,P4), 3 online brands (O1,O2,O3) and 4 mixed brands (M1,M2,M3,M4) selected.
Which have to be displayed and where?
I should say:
Split the question and ask separately these two types of brands. In this case the mixed brands appear twice.
Or use subquestion relevance in Q21 ff:
Insert all subquestions (physical and online). To physical brands show one part of the subquestions, to online brands the other part and to these mixed brands show all. To do this you should recode your brands that you have easy subquestion relevance equations
It's up to you.
Only you know what is the goal of your survey.

I am a bit tired to answer to always changing requirements
You started with

so my solution is to only have 10 fixed variables and show the ones that I have selected in Q13a,

Then you changed to

Of the selected brands, I am only taking the first 5 to apply my next questions, which are Q21 and Q21a.
In these two questions, the 5 brands that they selected in Q13a are evaluated, but Q21 only applies to physical brands and Q21a only to online brands.

And now

but I realized that there are many brands that handle both modalities, both physical and online.

And you really should read the manual and study the sample surveys which you can download there.
Just to avoid questions that only arise because you  are not familiar with the basics of ExpressionScript.

Joffm
 

Addition to the "mixed" brands.
Repondents who only used one channel can't answer the question to the other.
So, you should ask them, which channel the used (physical or online, or both).
This you can do by changing Q13a to an array(numbers) with checkbox layout.

And you should limit the number of brands in each column to 5. 
"Which of these brands did you mainly buy in the last 6 month? Please, select up to 5 in each channel."
Then everything is fine, clear and easy.

Joffm