This would work:
numcustomers = input('Number of customers ?');
customers = cell{1, numcustomers);
for customeridx = 1:numcustomers
customers{customeridx} = input(sprintf('Name of customer %d:', customeridx), 's');
end
Note, however that I personally dislike prompts like this. I'd rather define a function that just take a cell array of customer names and leave it up to the user to generate that cell array, through prompt as above, or a gui, or by reading a file, or just typing the whole array at the command line.
Imagine if you have to run your script ten times, where each time just one name changes. You still have to enter all the names every time which can get a bit tedious.