MATLAB: Open matlabpool error due to invalid configuration name

Dear colleagues, I am new to Matlab so I appreciate your help and patience. I would like to run "for loop" in parallel

I have text file (data.txt) contain the number of folders for the data that i aim to analyze. I would like to run the command:

      matlabpool open N % where N is the number of the entries in the file "data.txt"

I tried to run the following commands:

     fid=fopen('data.txt','r');
     list=textscan(fid,'%s', 'delimiter','\n');
     x=list{1};
     num_x=size(x,1);
     XX=length(x)

After then I ran the command :

    matlabpool open XX

This output the following error:

    ??? Error using ==> matlabpool at 125
    'XX' is not a valid configuration name.

Kindly what I am doing wrong? Your help is highly appreciated! John