Working with bode-plot figures is difficult.
A (simple) approach is to use empty strings for the rest of the plots
figure
hold on
for i = 1:10
sys1 = tf(i,[1 1]);
bode(sys1, 'b-');
end
sys2 = tf(20,[1 1]);
bode(sys2, 'r--');
legend({'', '', '', '', '', '', '', '', '', 'Sys1', 'Sys2'})
