I would say no, depending on what part of the country you're in. You say the property is 5 - 10 acres, but you don't say how much land is inside fence, which is really the key consideration.
You also don't say how you plan on keeping the horses. 18 horses with 5 - 6 acres inside fence is possible if: 1.) horses are stalled at least 50% of the time 2.) you have an agressive program of picking up manure in the paddocks and harrowing them 3.) you use a daily dewormer product 4.) you have excellent, tight fences 5.) you supplement heavily with hay and grain. And 6.) you have a regulary scheduled manure pick up or disposal off the property. It's a very expensive and high maintenence way to keep horses.
You might also want to check local zoning ordinances as well for what the maximum is that they will allow. In my area, it's a zoning requirement that you have three acres for the first horse, and an additional acre for each subsequent. That's total land, not pasture inside fence, but under that guideline, you'd need 21 acres minimum for your 18 horse.