how much portland cement

Wyokid is exactly right but worded it in a way that is potentially confusing. He means you need 5.5 90lb sacks of cement per yard, not 5.5 sacks totalling 90lbs. Cement has typically been sold in 94lb sacks because that's the weight of one cubic foot. The most common mix for a homeowner to have delivered is what's called a five sack mix--each cubic yard contains five cubic feet of cement. Cement is interesting because it is so fine that when mixed with dry aggregate, it occupies the spaces between the grains of sand. If you were to mix five cubic feet of cement with 22 cubic feet of mixed aggregate (sand and gravel) you wouldn't end up with a yard of concrete. Those proportions would make slightly more than 22 cubic feet of very rich concrete, just about a six sack mix.

Stan