Counting occurrences of a character in a string within a cell

I'm having trouble figuring out how to count the occurrences of a character in a string within a cell. For example, I have a file that contains information like so:

type
m
mmNs
SmNm

and I'm trying to determine how many m's are in each line. To do this, I've tried this code:

sampleddata = dataset('file','sample.txt','Delimiter','\t');
muts = sampleddata.type;

fileID = fopen('number_occur.txt','w');
for j = 1:3
    mutations = muts(j)
    M = length(find(mutations == 'm'));
    fprintf(fileID, '%1f\n',M)
end
fclose(fileID)

However, I get an error that informs me: "Undefined operator '==' for input arguments of type 'cell'." Does anyone know how to overcome this problem?