The main trail has 100 day trip permits, and 60 overnight permits. Of the 100 day hike permits approximately 90 go via the main trail, and 10 via the Mountaineers Route. If all permits are taken/filled, this suggest a minimum of 210 hikers on the trail, not counting the exit permits from other starting points. As a result, going solo on the main trail is fairly low risk. The trail is well marked, and normally there is someone reasonably close. However, last year there were two fatalities along the main trail that I know of. In both cases the accident occurred on the return leg of the trip when the hiker was tired, and perhaps not as alert as normal.
The Mountaineers Route is a different story. I "think ??" there are only 10 permits for overnights via this route. This does not count the permits available to the guides, but even that does not add huge numbers. Bottom line, this will not be a crowded path. Also, this is not a maintained trail, and it is fairly easy to get off trail.
So the answer as to how safe for a solo hiker depends on that hikers experience level. Unless you have experience route finding, hiking/climbing above 13,000', and are comfortable with some exposure, I would suggest the main trail, or consider a guided trip. Sierra Mountaineering International offers guided trips via the Mountaineers Route, and is well regarded. They also guide trips internationally. Using a guide service also takes care of any permit issues. See the following link for more information:
http://sierramountaineering.com/There are many trip reports via the Mountaineers Route, so do some browsing before you make a decision.
