Hi orangina and the rest of you going through the entry process for 2016 - good luck!
It's more a marathon, not a sprint so pace yourselves as the stress gets to you otherwise (easily said I know). Just when you think the open days are a challenge, you make your application decisions, then xmas comes before you know it and then they are straight into the madness early Jan, or earlier if boarding.
I think it's a personal decision how many your DC sit for. Only you can tell what their personalities can take or judge on their abilities, how many they need to take. If unsure, many over safety net (again, understandably). If this becomes you, try not to get drawn into the drama of stats of how many children sit for x number of places!
My DD loved the whole thing (I absolutely did not) and after a dodgy first exam day, corrected her approach on the rest (4 in total). She really wanted the chance to secure her ideal school. Other children were completely worn out, and absorbing some of the angst from their parents (natural) but they are still 10/early 11 so read your child's maturity/resilience levels before putting them into too many...whatever that number is for you.
Pizza/lunch always helps after each step if your junior school allows...many have said this process is tougher than GCSEs/A/Uni apps because of DCs' maturity levels but expectation to perform is enormous...
And read previous threads. Learn from them. They are full of knowledge and underlying all of the past years is the same message...most do get selected by the right school, many get a choice 