Q:

How many enemies can one Lvl 1 PC handle?

In the start of my adventure the PCs will not be together. I plan to have all them fight 1-3 encounters alone before finally meeting up. I read in the book about setting encounters but the minimum I saw was a party of three, nothing about being alone. They are of course all lvl 1. Their classes are Figher, Ranger, Sorcerer and a Cleric.
All the encounters will take place in a not so dense forest covered in fog.
Some more char info.
The Cleric has 13STR 10DEX 15CON 13INT 15WIS 18CHA
Feat is Quicken Spell
The Fighter has 18STR 12DEX 15CON 13INT 13WIS 10CHA
Feat is Power Attack, Cleave
The Ranger has 11STR 15DEX 16CON 10INT 15WIS 10CHA
Feat is Alertness
The Sorcerer has 13STR 15DEX 16CON 14INT 14WIS 19CHA
Feat is Spell Focus
I want them to fight animals, undead and maybe even demons if possible. The theme is mostly dark and dreary foes.

A:

I want them to fight animals, undead and maybe even demons if possible.
  The theme is mostly dark and dreary foes.

Are just the foes dark or is it a theme in your whole world?
If that is the case then kidnapping might be common and the rangers encounter could be with some peasants whose child has gone missing. It needs to be tracked down though a wilderness. Add a few brave people to help (e.g. the boys father and his brother, armed with pitchforks) and you have an instant mini goal with some NPCs as a buffer. 
For the cleric you may be able to do something similar pacifying a graveyard. The local priest or some of the family of the people who are burried there can help the their loved ones find eternal rest. This might involve turning them, or just keeping them at a distance while some rites are performed to clean the source of evil. This makes a great way to have an encounter without a fight.
The fighter on the other hand is build for fighting. 
I think that YogoZuno 's answer is quite good. Give the fighter something to easy kill. Adding an NPC or two or making the enemies quite slow will help. (Slow in case you have bad rolls and need to run away).  A nice tactical map might also help (e.g. one narrow point though which a few monsters must pass, and which can be blocked by the fighter. This will teach him that it is not just about hitting enemies, but also about placement on the map).
As for the sorcerer: Charisma 19.  That will come in handy with a nice political mission (e.g. convince the major of  a neighbouring town to aid another town or to convince a group of NPCs that they really want to do something else. Which, unless bungled, is yet another safe no-combat encounter. No risk of dying.

A:

Apart from YogoZuno's excellent answer, I've found that in practice the challenge a single PC can face safely varies a lot. With only one person fighting, a few unlucky rolls can be all it takes for a player to die. However, sometimes the player will do a lot better than expected and simply plough through the challenge.
I would suggest giving them one or two NPCs to temporarily help out. If they are commoners with sickles and staves, they aren't going to majorly boost your player's power, but they can drag him away and bandage his wounds if he is bleeding out.
Also, this is an opportunity for each player to shine. It can be a good idea to set encounters up to play to your PC's strengths - if you let the Cleric fight some low-HD skeletons instead of Kobolds, he'll have an easier time of it by turning them/using Cure Light Wounds to damage them and it will help teach him tactics for when he's in the party later on. If you let the fighter go up against a bunch of weak mooks (maybe animated piles of branches with a 1/2HD each who are clustered together) instead of a single, stronger foe, his cleave will come in very handy and he will start to learn to group his foes. This can teach valuable lessons to your players as well as characters.

A:

The results are going to be highly variable, and depend on the situation, and depend a lot on the critters facing each PC, along with the way the PCs are built. Any PC built to depend on someone else (for instance, the Sorcerer, depending on his spells, or the Ranger, if he is built for ranged combat) will struggle more, and should probably be allocated less opponents than the others.
The basic way to do this, though, is to take the XP allowance for a typical party at this encounter level, and divide by 5 (since encounters are based on 5 PCs normally). If you want an average encounter for level 1 PCs, this would be 80xp. A single raven or lizard are worth 65xp. Working the other way, a single Kobold (100xp) looks to be between CR 1 and 2 for a single level 1 PC, and a single Goblin is just over a CR2. A Hard encounter at level 1 is a CR4 encounter, which would translate to 240xp for a single character, which would be slightly more than a pair of Kobolds.

