It can apply to as many horses as you like as long as the odds converted to percentages do not exceed 100 when totalled.

An easy way is to convert the odds to percentages:-

2/1 = 33%
4/1 = 20%
6/1 = 14.28%
8/1 = 11.11%
Total 78.39%

Using the percentages as staking units 33 on the 2/1 shot, 20 on the 4/1, 14 on the 6/1 etc the total outlay will be 78 and the return approx 100 should one of them win, a profit of 22 units.

If you want to Dutch the whole field bar one then just lay the one you don't want to include (say 4/1), which is the same as backing all the others to win at 1/4.