25 to 30 mate, i bred 28 this year for the same size section, Not one flyaway, and finished with 26 after 1st and 2nd in the strongest club in the area, I have built bigger this year 9x6 and will be keeping the same amount, only for motivation so i can have pipos in it, the year before i had 25 in a 4x6 section with a 2 foot avairy that work just the same,,
Just keep them clean and fit and you will not have a problem with 25 to 30
Two mates also have 6x6 and are keeping them the same this year, also I lost the least young birds in the fed