One IP address can host a google of web sites (that's one before infinity). Company's have gone public (made millions and millions of US dollars) with the advent of virtual hosting. Once upon a time, every web site had its own unique IP address. The search engines actually thrive on link building in this manner, and have long ago become delirious in its combating link spam born this way. In today's world, practically every web site on the Internet shares an IP address. The question is - who do you share it with? If your lovely little 8 page business web site is sharing an IP address that is saturated with spammer and con artists then it will devalue your site for it. Remember about neighbourhood, Google used to suggest not to get links from "bad neighbourhoods. The IP address of a web site is an indicator of its own neighbourhood. Here's an example, if I have a web site hosted on a server, say, in Pakistan, and it hosts thity-five other web sites all sharing the same IP, wouldn't that appear to easily trigger for spam dampers? Let's say all that all these sites heavily interlink themselves and have amassed a whopping total of sixty-nine million scraper pages auto-generated to boot. Their actual content is data mined robotically and the whole network becomes one powerful scraper site whose exclusive agenda is to generate ad revenue (through either unavoidable click-through or outright click-through spam). Now, Do you think ... Do you think the search engine would appreciate this Internet marketer's web pages, and award them high value in a keyphrase search thus rewarding them with high rankings? :) ah isn't this fun, the dilemma of corporate greed over search engine results pages ethics and human suffering. So unfortunate. Unnecessary. Just about evil I would declare.
Yes of course, you can use a single IP across multiple domains. As canadafred mentioned, there are plenty of websites that share an IP address. Having a shared IP address across multiple domain names does not negatively impact your SEO processes.