Translation:There are a dozen of notes on my desk.
Or alternatively you could remove the "of", which may be closer to the Indonesian translation?
A dozen is satu lusin, exactly 12. Ada selusin catatan di meja saya.
Dozens is lusinan, many sets of 12. Ada lusinan catatan di meja saya.
I think belasan is any number between 11 and 19, not exactly 12.
I use the word "dozen" in English quite similarly. Sometimes it's clear that I'm talking about exactly 12 eggs, other times it's a bit like saying you have more than a few of something when "a few" can be a bit ambiguous too.
A bit like the word "beberapa"; it's all "kira-kira".