I have a normal full sized sixes stringed acoustic guitar, and because I don't have a capo, I usually tune my guitar up one or two semitones. What is the highest number of semitones I can safely tune up to on each string, which has a medium gauge for standard tuning?
Can't afford a capo? Can you afford a pencil and a couple of strong rubber bands?
I wouldn't tune up more than a couple of semitones. If you increase tension too much a string will break rather than the guitar. Probably. Don't risk it.
Here is a string tension calculator I've taken a liking to using. It has a handy explanation on how to use it underneath too.
Tuning up around 3 semi-tones is probably the limit of most strings. This also depends on the toughness of your neck and the gauge of your strings. Thinner strings will tune up more easily than thicker strings. Note that up-tuning tends to wear out the metal around your tuning machines since it has to bend back and forth many times along with the increased tension. It can snap at the top if you do it too much.
Can't understand why you'd want to tune up a semitone or two. Far more people are tuning down these days!
It is very dependant on the guitar in question. I've seen strings tuned up an octave, without breaking, but that's not a recommendation! A tone will most likely be o.k., but it's putting more strain on the neck than it's designed for. The bridge may get pulled off - seen that too many times - and it's going to be hard work pressing the tighter strings down.
If you want to play stuff in a higher key, either get used to barre chords, or, if you prefer open voicings, invest in a capo, or, as Laurence suggests, use a pencil and rubber band. Yes, it does work - and it's not going to break the bank!
I'd recommend that you buy a capo. Failing this, it depends on your strings - higher strings might snap faster than lower strings when tuning them up, and different types of strings behave differently (how are they coated? is the G string wound or not? etc).
If you insist on tuning up, I wouldn't risk more than maybe a half-step or two. Whatever you do, make sure you keep an eye on your neck tension as you apply more pulling force to it due to tighter strings. You don't want to warp your neck through unnecessary strain.