Well, their assets are begin served from a CDN. Are they counting that in their server count?
$ host assets.tumblr.com
assets.tumblr.com is an alias for
assets.tumblr.com.edgesuite.net.
assets.tumblr.com.edgesuite.net is an alias for
a1092.g.akamai.net.
a1092.g.akamai.net has address 69.31.106.32
a1092.g.akamai.net has address 69.31.106.50
$ curl -i 'http://assets.tumblr.com/images/favicon.gif?2 HTTP/1.1 200 OK Server: nginx/0.8.53 Content-Type: image/gif Last-Modified: Fri, 15 Apr 2011 22:13:30 GMT Accept-Ranges: bytes Content-Length: 635 X-Varnish: 1795992965 Cache-Control: max-age=2196224 Expires: Sat, 10 Mar 2012 05:35:30 GMT Date: Mon, 13 Feb 2012 19:31:46 GMT Connection: keep-alive
So it looks like their application is being served by Apache (and PHP), while their static assets are served by nginx behind Varnish.