GFP gene number vs. fluorescence

[ara]

Would you tell me how many GFP gene (pEGFP-C1) do transfected cells need
for us to detect fluorescence? (I used CMV promoter and HeLa as
transfected cell : I don't mind any cell type :) )
I think the transfected cells should need over 1 number in order to
satisfy our dectection.

Thanks in advance..