Allotments 4 All

Hello all, I have a three-year-old Boskoop Glory grapevine growing in my polytunnel and this year it has gone madly insane with small bunches of grapes all over the place. I have trained the vine to grow up and across the full length of the polytunnel over its first two years. In the second year I allowed one bunch to develop and they were so sweet it wasn't real! Now,in year three, I am told I can allow much more fruit to grow on the vine,but how much is 'much more'?  Anyone with advice out there? Thank-you for your time :)