For myself, I use a separate drive for my projects, and another for samples as recommended. Also several externals for backups.
 
But for a friend who wants to get started with is single C drive computer, how much of an issue will it be to run that way?
 
Is it a situation that at a certain track count it get's tricky?   I guess it would just be easier for me to recommend an additional drive if I could explain why it's a good idea.

Assuming the C drive is a spinning HD, audio track count will be limited but may be enough to get started. Definitely would set the expectation that a second drive will be in their future.

Part of this also can be mitigated by work flow, since a clip is a window into an audio file, so it is easy to have a track consist of hundreds of piece parts this way. I would get him started with a habit of bouncing/archiving as he works, as reading one audio file is substantially easier to the system in general.
 
If it is a HDD, defragging regularly is also a good idea. Load times will be slower (each file read/written must occur in sequence due to the heads being physically linked), but being new he would not notice this.
 
If an SSD, the only real issue is space (as audio can consume space quickly), so good housekeeping may be required, depending.
 
As scook mentioned, it may be easiest to simply start him off on the right foot with a second HDD, even a simple 1TB drive would be significant.

A lot totally depends on what that drive is. 
If it is not an SSD or at least a 7200 RPM drive then issues might arise. 
But I worked without issue for a long time with a 1 Drive desktop but it had a 250 Gig 7200 RPM drive. 

interesting, thanks.
 
All I know is it's a spinner, one would assume 7200. I would also assume a internal drive would be much better than an external usb drive.
 
If track count is the main issue, it would be interesting to know at what point, how many tracks would typically start to affect performance. It may be good enough for small projects.

Internal drives are preferable. If the drive is not a fixed 7200 or higher (no green or energy saver types), it will be trouble.

A decent dedicated 500GB "Audio" HD can be added for $50.
If this person is serious on any level (even as a hobbyist), it's worth the investment.

Wow, there are a lot of prophets of doom on this thread so far.  At home, I run an SSD for my OS drive and samples and a separate WD Black drive for all my audio/projects.  This is what I would recommend to anyone.
 
However:  I had an instance a couple of years ago where I had to record some hearings, with a minimum of 6 channels simultaneously via an M-Audio FastTrack Ultra.  The only thing I had available was an old laptop with a very average single HDD installed.  It was also under-sized to the point that I had to copy off the recordings every couple of days to a USB drive.
 
Long story short, I recorded several days of hearings, many hours recording straight per day, and never experienced so much as a glitch.  Do I recommend this? - No.  Obviously I wasn't trying to play back a bunch of soft synths or overdub large numbers of tracks, but I have to say that I have been surprized how well Sonar works on modest hardware on more than one occasion.
 
End of the day, no it's not ideal, but it will work.  Given the junker I had to work with, a modern machine with a reasonably fast HDD should do the job.  Nothing wrong with this as a starting point.

In a one-HDD system I'd say the most important thing is to have an idea and a plan of how to do things.
If the overall workflow isn't organized things may end up messy and glitchy.
As already mentioned above: 
- Figure out the energy efficient workflow per Project - when to bounce soft synths to audio and a multitude of clips to whole audio clips. It's also best to forget VSTi's that run heavy sample libraries and the likes.
- Figure out the safe and energy efficient workflow per your PC. When to move projects/material to an external device
to make room, and when and how to make backup-copies of everything.
 
IMO, one does need an external device or two anyway, because you shouldn't operate without a proper backup plan, and most likely you'll need to remove stuff that you don't want to delete.
Whether those are DVD-Rs, memory sticks, HDDs or SSDs is another story.