I am buying backlinks for someone for 5 of my websites, I have read things that say the more backlinks the higher the rank, but I have also read things that say, if you link too many pages, too quickly, then your page will get deranked, how do I know how many links to build / buy and how fast so that I can get ranked without Google dropping my website.
It depends on what kind of links they are to be honest, if you simply spam tons of blogs with comments, then you'll get noticed for spamming very quickly. If you spend the time to build high quality links, then you won't need as many links to see an increase in rankings. It really depends on your time scale, how much money you have to spend on link building and how quickly you want to see results from it.
I want quick results, but I also want them to stay, so I guess I need a mix, as for how much I have to spend, it should, in theory, pay for itself very quickly no matter what the amount that I spend on links is, because with SEO comes higher SE rankings, and with higher SE rankings comes more traffic and more sales. If I am buying links, how do I know if they are crap links or HQ links?
Who buys 5000 links at once for a new site these days? Firstable check if the site where you get links from has high trust rank. Check it with opensiteexplorer by clicking "compare link metrics". If site has so called "Domain MozTrust" (not "Page MozTrust") higher than 4,00 the links will have real power. Also mind that it's good if, according to OSE, site has Page Authority and Domain Authority higher than 30. For instance: http://www.opensiteexplorer.org/comparisons?site=triumvir.org
Do article marketing, social bookmarking for and etc for a fews day..then, after that you start buying backlinks, a lot of backlinks
Try buying a 100 for now and then match that with a 100 links that you built manually/naturally. These days, buying links are not as effective, so you need to balance the bought links with natural inks. Hope that helps. Have a good day!