How many articles can be submitted using one Web 2.0 site account?

You can submit more than one article in day using on Web2.0 site account in a day but try to submit articles in limit because if you submit 5- 6 article per day then sometimes it become over optimization of content and it is quite possible that few websites can ban you as a spammer. So my opinion is to submit 1–2 article per day on web2.0 sites.

 You can post the single unique article content only one time. Don't try to post the same content on multiple blogging sites.

If you do the same then google will ignore that one because Google counts only quality backlinks.

You can submit multipe articles on web 2.0 site. But there is best technique to create traffic from web 2.0 is that you have to submit one article per week.