To keep the autoranging limits if they're ok and using a tick spacing of 100, but you want 200, simply
ztk=get(gca,'ztick'); % retrieve current tick locations
set(gca,'ztick',ztk(1z1:2:end)) % setting new ticks at half spacing
If you need to readjust the start value, you'll have to query the min/max from each data set and determine a suitable zlim range and ztick spacing but sounds as though you're ok with the auto range, just want to change the number of ticks...above will reduce by a factor of two beginning with the first; if it were an odd value and you wanted the evens, you'd have to check the value of the first element in ztk and see if want to start with ztk(1) or ztk(2), that is
iz1=mod(ztk(1),200)==100+1; % first index in default ztick divisible by 200
Demo: try
surf(20*peaks(50))
ztk=get(gca,'ztick');
iz1=mod(ztk(1),200)+1;
pause(2) % delay to see initial plot before
set(gca,'ztick',ztk(1:2:end)) % reset z tick spacing
