Dunno where you live, but the pan-London system may be similar -
You put down your 6 choices in rank order and send to central admin
Central admin sends your child's details out to all schools (not telling order)
Each school rates you according to their criteria and offers a place / not and sends back to central admin
Central admin looks at your rank order and puts down your childs best offer, then tells any other schools who may have offered a place to "cross you off" their list
Schools who have spaces because of this then look at names on their lists and offer again, etc
When every child has got a place, they send the offer of the place to you!
Whew! So if you only put one down and don't get it, you will be offered something else. You can, of course go on the school you wanted's waiting list and hope for the best / appeal.
So if it was me, I'd try to find as many possibilities as I could and be prepared for waiting lists, etc.