From where I'm standing, the old way of (ab)using Web 2.0s is dead. If you want lasting results you need to create web 2.0 properties that attract traffic from both the search engines and the main website.

For instance, if you have built a Wordpress.com blog, don't submit a crappy article for the single purpose of having your links embedded in the post and leave it at that. Submit stellar content, interact with the other Wordpress.com members that have blogs in your niche. Ask them to contribute in your blog, contribute in theirs. Send meaningful comments. Make friends, get blogroll links .. Make your Wordpress.com website a traffic magnet.

Same goes for ANY web 2.0 property. You have to be build an active community around the Web 2.0. That's the only way of doing it and passing trust, page rank and traffic back to your site.

That's the only way you'll have results long term.

If you're just spamming web 2.0s you'll only get 3-4 days of fame (if you're going to get there at all), then your website will be penalized and you'll never going to recover from it.

- Gabriel

web 2.0 backlinks ? we need it but it is not a must have
howmany ? 120 is enough

Without knowing the OP's site, niche, competition etc then giving a specific answer is pretty meaningless.

In general a fixation with WEB2 as first tier is not a healthy one. Any site with PR and traffic that allows a link - preferably with content for context is a good first tier site. Slightly favouring D0follow also makes sense. Whether that site is WEB2, article, WIKI.... really doesn't matter. It's not 2009 any more