One per 70? Perhaps if you are running a ton of clone servers or heaven forbid working at an ISP handling boxes for customers.
In the *real* world one normal admin per 35 or so is considered more normal for unix staff (with a senior admin for every 3-4 normal admins). While I have seen the numbers quoted by the parent they were very for generic servers in function and basically tons of copy (virtual) boxes. In addition if you are running a good deal of unix servers you more than likely have a lot of different functions spread out among clusters, one person responsible for all these apps as well as keeping the boxes well maintained, troubleshooting upgrades, etc... can be quite tiring. Unix boxes these days tend to do significantly more than they used to. On the flip side, managing unix boxes these days is quite easy thanks to ssh/cfengine/puppet other automation.
When I first started doing unix for pay, mid 90s, we had one admin per two servers which was a mirror of the typical university environment where there were single admins for important machines and tons of lackeys for all the sun/hp/dec desktops. By the year 2000 this had grown to around 20 servers for each admin, with senior staff expected to be able to handle functions across any cluster. These days if it is a smaller company, there's one senior admin, who may or may not be the Network/IT manager and they typically handle 30-40 servers if there are a number of functions spread out. Of course having tons of similar boxes makes this number much higher. The most I've ever handled was 165 servers spread across two admins, three sites, four major applications, tons of clusters, including DB/Application/LB/Development/etc...
Also, if you have 70 unix servers and only one admin, the point of failure for your organization is beyond catastrophic. That poor lil' admin guy is going to get many of the "what happens when a bus hits you" questions from 3rd parties/partners/other departments.