for all numbers and sometimes demonstrative pronoun 一座山，2个人，三朵花，4栋房子 那个人，这辆车 ect

“量词” is confusing even to some Chinese child

sometimes we do not use 量词 for numbers if the word is a noun.e.g.三轮车 双人床 double bed 双means two. e.g三人行必有我师 a chinese old saying.e.g. if you booked a ticket for one person.how much for one person? 一人多少钱？or 一个人多少钱?are both ok

all numbers including 一 u must use 量词 furthremore, for 许多（some） and other words like this u must use 量词 too

yes what you said is right.一个苹果 两个苹果 三个苹果is like one apple two apples three apples.and the only way to handle them is accumulating.good luck~