Ah - Japanese numbers - they drive me crazy! Given that there is no rhyme or reason, you just have to doggedly memorise them, much like we learned our times tables (or at least we did when I went to school). Be sure and nail the correct way to count people, particular one person and two people (ひとり ふたり）.Be careful of 4, 7 and sometimes 9 as they change form depending on what you are counting. Just accept that they are, shall we say, idiosyncratic and drill yourself on them until remembering them isn't such a struggle. When I am talking to one of my language partners and come to something where I need a number, they always sit patiently with a small smile while I struggle to dredge up the right form, then either applaud energetically when I get it right, or gently correct me when I get it wrong.