You're performing matrix division instead of elementwise division. Use ./ instead of / in your Y calculation.
Once that's fixed, you'll get one plot and only one, because by default plot erases previous plots. Use hold on to prevent that.
Also note that since X doesn't change with m, it'll be faster to only calculate it once, outside the loop. So:
figure;
hold on;
X = linspace(1, 100, 1001);
for m = 1:10
Y = (X.^m) ./ (5^m + X.^m);
plot(X, Y);
end
edit: also note that you can avoid the loop entirely (and the need for hold on):
figure;
X = linspace(1, 100, 1001);
m = (1:10)'; %as a vector in a different direction than X
Y = (X.^m) ./ (5.^m + X.^m); %requires R2016b or later
plot(X, Y);
