With so many pages on the web,
quality will usually win over quantity. With that being said, sometimes it will
make sense to have multiple, similar websites covering slightly different
topics. Doing this can help you create topically authoritative inbound links to
different sites in your network and give you a multi-branded approach to
marketing.
However, you want to make sure
your sites are all different and unique. If your sites are extremely similar, then
your sites may receive a spam penalty or have the nepotistic link popularity
discounted. Even worse is that, if you interlink them all, then all of your
sites could get penalized at the same time. Those using strong brands and good
ideas can usually do well without creating a topical network. If you create a
topical network expressly to deceive search engines, then you are taking a risk
and your sites may get removed from the search indexes.