oldcowgirl said "I was told many years ago whether it is true that legally all they can ask is how many people live at your resident. As far as finacial questions I fill out a tax return so the government already has that info so they should not be asking. "
So, Statistics Canada can ask you a lot more than that. The Census Act basically says they can ask any question deemed to be needed. Not everyone gets the long form (most people get thr short form that more or less asks exactly what you state), because they don't need it to get a statically relevant sample (and it also becomes cost-prohivitive).
Census is more than just counting people. It's understanding who we are - our heritage, language, sexual orientation, our housing, how and where we work etc. It's one tool to help us know how programs get funded, who needs more help, what general trends are etc.
All information is confidential. Your tax returns are also confidential, which is why that info doesn't get shared with Census - the Income Tax Act prohibits it unless a MOU (Memorandum of understanding) is in place. There are lots of layers of security and protection to keep your data safe.