You are trying to create a result matrix which stores the max value of each row in each record. In what you are trying to do the result matrix is being recreated again with zeros for every edfread. You can create result cell array for each record instead of a matrix. This way a new cell array element is created for each new file being read and the previous entries are not lost. Refer to the code below:
result{ii} = zeros(size(record,1)); % Instead of result = zeros(size(record,1), ii);
You can then use this specific entry for current edfread in the nested loop. refer to the code below:
result{ii}(jj) = max(X); % In the nested loop, to store the max for each row instead of result(jj,ii) = M;.
