how many eggs should i be getting?

hi,

my chickens just dont seem to be laying at the rate i think they should be, there are 20 hens. Most are pure breeds ( astrolope, cochin, legbar, arancuna, marran) but there are some warren, blackrock as well.
most days im only getting max 10 eggs and today only 6!

they are all 12 months old and at times i have had more eggs from them.

they have 24 hour access to layers pellets, fresh water, grit and shell and get a bit of corn for a treat. They all seem healthy and active.
am i expecting too many eggs? Is there a reason why they are not laying more eggs?
 

different breeds lay different numbers on average.  Some breeds will lay 5-7 per week, some as few as two.  the numbers you are getting are not therefore unusual.  If you need lots of eggs you need to select breeds that are more prolific layers (if you google frequency of egg laying you will get a list of breeds)

My bantam hens go through low production periods but the way I see it is that they will be laying for longer. 

As Joy said, weather does play a part in which way they go, but the warrens and blackrocks should still be laying a nearly every day.

Also pure breeds may lay consistently for a few weeks (even if it is three or four eggs a week) then stop for a few weeks, then start laying again etc.

This is a more natural laying pattern for hens, rather than the lay-every-day hybrids which are comon place now.