..absolutely and Google will never give guidance as "buying" links is, to put it mildly, frowned-upon! ..there are too many variables - but as someone said above if you have good quality content and huge readership then you'd get away with throwing more links at the site. Though I'd argue then, why would you want to. ..my view is it's much easier (& safer) to "build" links by having something interesting to say and acquiring them naturally, than it is to build them in forum posts / signatures etc. Most "built" links are off-topic and on poor quality sites. Do you think Google really doesn't know the difference between a built forum/signature post and a "genuine" link to your article from a well regarded website. ..and always remember "cheap-links" are cheap for a reason Hehe sorry if I come across as a cynic. I'm sure those selling link-building packages will disagree with me!
No definitive guide on this.It depens the age of your domain..if your domain name is very new..then start with 5 to 10 links a day...