Hi @utsavchopra, web 2.0 is pretty cool indeed.
You can create only one Tumblr account via Yahoo! ID, but you can create sufficient number of blogs in that Tumblr account.
3-4 links as long as they look natural mate. The trick is to forget that they are money sites.
I wouldn't honestly suggest this bro. If it looks like spamming, then you need to reconsider your money site strategy. It's not just about building backlinks, it's about relevancy as well.