There is a lot of variables in your question. My experience is that if you have 20 different blogs interlinked together and they are all optimized for different keywords and niches you may be fine with having them all on the same IP. If you are making 20 blogs that are all going to be in the same or very similar keyword you may find a couple of them getting ranked high and the rest not ranked very well at all.
If you are going to work in similar (or the same) keywords at the least you might consider getting different IPs for the sites and you might give
www.gotwebhost.com a look to see what the other options for you are. If you are going to work in 20 different varied keywords, you may need to do nothing.
The thing about SEO is that there are a ton of ways to do it and many ways to have success. Most of it is trial and error.