If the strings will have different length, you can not do it. Instead use a cell array:
amount = 3; % or whatever you want
my_cell = cell(amount,1); % empty 3x1 cell array
for k=1:amount
tmp = sprintf('String%g',k);
my_cell{k} = tmp;
end
my_cell =
'String1'
'String2'
'String3'
