Completely determined by the circumstances. If it's well publicised and the block is on sale to home fans straight away, nearly the whole block will be full of 1882 (especially towards the back). So in block 35, what's that? 500?How many on average attend the 1882 designated 'block'?
However Nottingham Forest, block 35 was originally an away block and it only went on sale to home fans at the last minute. I'd say there was 150 max who were there specifically for 1882.
Woolwich U21 away in 2013 was publicised for weeks/months before and there were over 1000 there. But then there's the odd youth game at WHL where it's literally 40/50 people there just having a laugh and singing funny songs, rather than creating an atmosphere (e.g. Southampton the other week).