At 20 weeks there is no guarantee that any of yours will be laying yet. Some of them do start that young, but a lot don't.

20 weeks = 5 months.I'm starting to panic and I don't know what to do! My hens are approximately 20wks old and I still haven't food one egg. I have five Barred Rocks, and two OEGBs,

Years of us raising new additions and once in a blue moon has a young pullet produced an egg at 5 months old. The majority of my newbies start closer to 24 weeks or sometimes later.
A watched pot never boils.
"The waiting is the hardest part"
Tom Petty