There is no reason if you are getting overs on your selections that you can't back multiple runners.
If you rate the field and find that you can get overs on four horses in the race, I don't see any reason why you shouldn't back them all. Unless your ratings are up to shiit....

Do not as previous posters have mentioned, just increase the number of horses you back just to increase your strike rate. If you are getting unders on the horses, then all you will achieve is to go broke quicker. I guess though, this may be a good thing if you have a designated bank, and will stop after that bank is gone. After all, then you will have more time to do other things than lose money.

My opinion is that you should dutch any selections. The shorter priced horses will always win more times than the longer priced horses. So you should bet that way.