None of your displayed variables are used within the load and they won't work in this way either - starting with LET and assigning then an aggregation-function. Here you will find many valuable informations about Variables.
I guess what you need is to do this:
SET numMutues = 'Count(nomMutua)'; // SET function defines a string which can be used in the script, you must use single quotes in this case.
SET rangs = 'Floor(255/$(numMutues))'; // dollar sign is needed to exchange the variable name by its content
But you will need to add a GROUP BY statement after FROM in your LOAD script, because you are using the Count() funcion. So something like this:
LOAD ApplyMap('mMutues', "ID", 'No informado') as mutua,
$(rangs) as ValueRanged
GROUP BY "ID";
I do not completely understand what you want to achieve but hope this helps.