To all of them.
At 2nd level, whenever a magus casts a spell with a range of “touch” from the magus spell list, he can deliver the spell through any weapon he is wielding as part of a melee attack.
Note that this part does not mention the free touch attack granted by casting a touch spell. Meaning that even if your free touch attack miss, or you decide to hold the charge, you can still attempt to deliver the spell with future melee attacks.
The spell will be discharged on the first attack that hits, just like it happens with the normal touch attacks. This could even be an attack of opportunity.
If you are a wizard and you cast Shocking Grasp and miss your target, you can keep trying again, if you have multiple attacks, you can attempt to touch with each of those attacks. The same is valid for the magus. Spellstrike allows you to deliver touch spells with melee attacks.
In the case of Chill Touch, you can discharge it once per attack, limited to one discharge per level. But keep in mind that this spell is vague and the Duration can be either until all charges are used or all attacks you can make this round. And the Effect can be either you can deliver a charge per attack or you can attempt one attack per level as part of the casting, allowing you to deliver 14 attacks in a single cast if you happen to be 14th level. So be careful when applying RaW to this spell, check with your GM how he would rule it.
To be clearer, say you are a 9th level magus, which has a +6/+1 BAB, you just got your secondary attack. With Spell Combat, you can cast a spell on one hand, while still attacking with the other, just as if you had an offhand weapon and were two-weapon fighting.
So, if you cast a spell during a full round attack, it can be cast either before or after the attacks (but not between them).
Also, with Spell Strike, you are allowed to deliver your melee touch spells with melee attacks instead. Replacing a non-damaging touch with something that will hurt.
- Cast Shocking Grasp, and then deliver it with a free melee strike.
- First attack from your full-attack.
- Second attack form your full-attack.
For a total of three melee attacks. If the free attack misses, you could use the Hold Charge option and try again on the next possible attack, meaning you could attempt to discharge it three times in a round. All of these attacks suffer -2 due to you casting and attacking with spell combat, just like happens with two-weapon fighting.