Let's say, if I have 100 or 200 or even more forum posts. Adding new site in the signature would get me 100 or more backlinks at once. Is that to much to do at once? When to do it, on the start of blog or later after I already have backlinks. Could that be dangerous for SERPs later?
No it's not harm any way it will give you profit, Yes you can see some changes on your SERP it will rank high on search Engine
Just because you add the backlinks in the forum signature does not mean your profile page is indexed. As Google gets around to finding and indexing all of your profiles it won't happen all at once. Therefore... it should be no problem
I wouldn't recommend doing it all at once. Because even though it's to the same site, it's going to throw up a red flag that google might not like. Do it gradually, say 10 a day for 20 days.
Dont worry, the site will be OK. The worst thing that will happen is it will plummet for a little before moving higher up.
Even if you change your signature. all the pages ( which contains your posts ) are not going to be crawled ( or re-crawled ) in a single day. most of those pages would be old enough and wont be get crawled. Even if some pages would get crawled, the link juice benefit would take time. so don't care.
I've seen some sites plummet and google dance before moving higher. 100 or 200 isn't too much in my opinion. If you're trying to pull higher than that like maybe 500 to 1000 then that might be a concern.
No, this is a common scenario and people sell their signature links all the time too with many more posts than 100-200. Google can handle this! Heck, some new VBulletin forums have your latest blog post showing on every profile, so your new blog post gets a link in your profile box on every post.
People do it because they are desperate, and do not know better. I call it the blind leading the blind or wow, everyone is doing it, so I will do it and get rich too. I doubt anyone will benefit that much from any signature link, a few may get indexed, but will go no futher then that...google is not stupid.