How many links per month is the best way to link to my site without to hurt my site reputation in google. I heard that if you make to many links google will ban you it must link constantly. - by posting articles - by buying PR links - by posting to relevant forums
There isn't a set number. Do some research on your competitors in your niche. Look and see how many backlinks they have and how old their sites are. That should give you a number that will be acceptable. For example, a site on yarn isn't going to get many backlinks (normally), but a web 2.0 social networking site that gets dugg or slashdotted will get thousands in a week. It's more about the quality and trust of the link anyway, you could get 10% of the number of links your competitor has and still rank above them.
If you gain links quickly and by the masses (for example, 100 links in 3 days and then 5 links the rest of the month), you're asking for Google to sandbox you. Google won't penalize you for getting inbound links. Google's algorithm tries to make sure that no can one cheats the system by purchasing hundreds of links and then coast the rest of their life from the SERPs they're getting. In simpler terms, Google is going to make you wait and have patience for high quality SERPs.
That isn't necessarily true, otherwise practically every site that gets dugg for the first time would get penalized.
It depends on the type of links you gain. 100 directory links is bad. 100 links from blogs (content areas) is good.
I see what you're saying, but I wasn't referring to being penalized - unless you see being sandboxed as a penalty. My point was that if you gain links quickly and by the mass, it raises a red flag for Google.
I see sandboxed as a penalty, it drops you into oblivion. It probably will raise a flag, but if they are legitimate links then there is nothing to worry about. If they're a bunch of paid links on irrelevant web sites or a bunch of directories that's a different story. Like mad4 said, quality editorial links are the way to go.
How long has your site been in the google index and does it rank top 30 for any terms other than the name of the website?