Not as many as all that. The downtime for DH for maintenance and upgrades is very negligible when compared to the volume of CERT advisories that must be addressed.
And, believe me, other hosting services are NOT as diligent as Dreamhost, nor as secure, nor as conscientious about fixes. Other hosts are down a LOT more, not for scheduled maintenance, but for other issues because they aren't as diligent as DH.
And, yeah, this is from personal experience.
I have to work with a hoard of hosting services of client's choice, and I can tell you that the majority are VERY unstable, down a lot more, and take forever to respond to issues.
zentao web design, graphic art and design at www.zentao.com
zentao7, Gallery of Artists and Speculative Novel Writers Groups