The movements kind of depend on you, too. Such as what you're eating, how you are exercising, if you are walking a lot. I read in a magazine that the doctor gave me that the babies are asleep 98% of the time in the womb so it might just be lazy and asleep! If you are concerned though, call the doctor.
When i was 21 weeks mine stopped moving for 2 days. Wrnt to the dr and she was totally fine (just a lazy day). Dr said its completely normal at that point in pregnancy
Yes at 20 weeks your baby is still small. He's most likely moving you just can't feel him all the time