I have a article directory (wordpress plugins), and now I decide to submit (or let people submit) around 50 articles daily, I want to know if google will sandbox my directory since I heard that more than 50 wordpress posts daily will get your site sandboxed for sure..
yes, you have to be careful, take it slowly, make your site look natural when increasing the number of pages. Otherwise, you will face the "sandbox" effect issue and you have to wait a couple of weeks (possibly 1-2 months) until you are out of that status. I had a similar problem with a new directory of mine - www.e-guy.info - so I stopped adding new content for a while. A friend of mine, had a similar issue and had to wait like 8 months!!
I dont think that 50 posts per day will get you sandboxed if you have a lot of indexed pages. (think about big news sites - they post a lot more than 50 news per day) However, you might get sandboxed if you have only a few pages already indexed by google.