OK. I voted (and said 1). But I'm not sure what the actual number is. It depends on how you define HTPC. My answer may be none. I think that a better answer would be that I have 1 (mostly) integrated (or on its way to being integrated) media system.
For most HTPC people, the playing of video material (from files, dvd, etc.) is a defining feature of their HTPC. This is one thing that I do not use a PC for. I'm currently using networked devices to play files or actual, real-life DVD players to play DVDs.
My integrated media system:
1) PC dedicated to PVR - recording and temporary storage only. Rarely used for playback. This is the only PC in the living room with the stereo/TV/etc.
2) Second PC
a ) file server - all mp3s and video files accessed by the rest of the system.
b ) winamp for mp3s - this is my primary mp3 player. An optical cable connects the PC to my living room receiver. The web-based interface to winamp is provided by Browseamp.
c ) XM satellite radio - my newest acquisition (to play with, rather than necessary). The audio from an XMPCR device feeds into a sound card on this pc and is selected through winamp and run through the same optical cable as 2a. I'm still working on rigging an interface for this. The source selection is handled through my browseamp interface but I can't change channels yet.
d ) media servers - this machine is running servers for MediaMVP and GoVideo 2730 networked devices in other rooms.
e ) this is also my HomeSeer computer and is running my apartment.
3) Networked devices in living room and bedroom connected to "stereo" These devices are used to play video from the server and rarely used for music or photos. Why do I have two of them (and in the same room) - no reason, just wanted to play with both.
a ) Hauppauge MediaMVP - audio, video, and photos
b ) GoVideo D2730 networked DVD player - similar to MediaMVP but also plays regular old DVDs.
c ) Audrey running PhJuke - this is the primary mp3-playing device in the bedroom. The output from the Audrey is connected to a receiver. PhJuke finds mp3s on my server through a shared directory (\\server\music).
4) Control of system - the media system is primarily controlled through web pages running on Audreys throughout the apartment. The same web pages can also be accessed wireless on touchscreen machines or a wifi PDA. These are done through HomeSeer - which is also used to transmit IR through a wired system. None of the interfaces looks particularly fancy (I wouldn't use them to sell the system), but they work and I can move on to other projects. Some things still need to be done with normal IR remotes (and to be honest, even things that could be handled through the web pages are often done with remotes).
5) Distribution - the output of the living room system (audio and video) is distributed throughout the apartment one way or another. There is a direct connection (composite video and audio) from the living room to the bedroom (one way only) so anything playing or selected in the living room can also be played in the bedroom (unless it's a digital source - my receiver will not put out line-level analog from a digital source (this includes my optical connection to the winamp machine, unfortunately)). In addition, the output of the receiver is also fed to a modulator - cable channel 100 in my apartment is this distributed signal. Also, the second ( B ) speakers from the living room are run into the office (where the other computers are to begin with).
This, of course, does not include the "workstation" PC that I'm normally using for other things and rarely use for this stuff. However, video files and DVDs played on that machine look great with its monitor. I wouldn't mind having the monitor in the living room.
That is a long (too long) explanation of why I'm not sure if my vote of 1 is the correct vote. By most HTPC people's definition, I think maybe I have one - even though I don't really use it to watch video. By other definitions, I have 2 machines primarly used for multimedia stuff in one way or another. If you count network devices that are running linux under a front end, you'd need to add a few more, but I don't really think that counts.
So anyway (again)...
I think my vote of 1 is a vote for a single HT system, not a single HTPC.