Roger tramples through the littluns' sandcastles quite deliberately because he enjoys causing pain and suffering to other boys. Roger is a sadist who gets a kick out of being a bully. Maurice follows him because he is also a bully, but unlike Roger he is also a coward. He is happy to follow Roger's lead but lacks the nerve to do things by himself. Even while kicking down the sandcastles Maurice is mentally preparing an excuse in his mind.
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Maurice is a bully but he is also a coward. Maurice hasn't got the necessary focus to commit acts of evil on his own but he is more than willing to be led. Maurice is happy to follow the lead of Roger or Jack and actively participate in whatever excesses they devise. Maurice is a 'heavy,' he is a thug who carries out the orders of others but he hasn't got the imagination to think up evil by himself. Maurice is a fox led by wolves.
Roger and Maurice.
After he'd followed Roger's example and trampled the sand castle of the three littluns Maurice hurried away and to quote from the book... 'In his other life Maurice had received chastisement for filling a younger eye with sand. Now, though there was no parent to let fall a heavy hand, Maurice still felt the unease of wrong-doing. At the back of his mind formed the uncertain outlines of an excuse.' Essential Maurice had been punished for hurting at least one younger child in the past and knew that he had just done something wrong again. So, he was guiltily preparing an excuse to try and avoid a punishment which wasn't going to happen, as there was nobody around to call him to account.
Roger like all boys had probably been told numerous times by adults not to do things. "Don't throw stones or you might hurt someone," would have been drummed into him to such an extent that even though there were no adults present to stop him or chastise him Roger subconciously obey their conventions and threw the stones wide. He probably wasn't even aware that he was doing it, he probably really wanted to hit Henry with the stones and hurt him but he simply couldn't do it, not at that point in the story. Later on, as his true personality blossomed and he could behave exactly how he felt thanks to the power of face paint to divorce a boy from his own actions, Roger was able to tumble a huge boulder at Piggy with no regards at all to conventions or the possible hurt he was likely to inflict. He could do it simply because he wanted to and there was no longer anyone or anything that could hold him in check and stop him.
The exact number is never mentioned. When Ralph asks Piggy the same question Piggy replies, "they stayed for a minute or two after you left to climb the mountain and then they all drifted off to play or to go into the jungle and I couldn't count them all."