It's easy to travel alone in Cambodia, and plenty of backpackers do. There are plenty of opportunities to cross paths with other people, but singles travel is perfectly possible too.
In terms of travel safety in Cambodia, violence towards foreigners is relatively rare, but exercise the normal caution while travelling alone. The biggest threat to solo female travellers in Cambodia is bag-snatching. It's unfortunately very common for a woman riding a bicycle to be accosted by a motorcycle driver who drives by and grabs her handbag, sometimes dragging her off the bicycle in the process. Therefore, it's advisable to keep valuables in a concealed money belt, and carry the bare minimum of valuable possessions at any time. If renting a motorbike, stow valuables under the seat while driving.
The night bus can also be a hot spot for assault and petty theft, therefore if taking the night bus alone, use extreme caution. Keep belongings close and try to get a single bunk near the front of the bus. Risk-averse travellers should skip the night bus altogether, as the day bus is much safer.