How does one know how many downstrokes and upstrokes are needed to play a song. For examples: If I want to play Bryan Adams' "Everyting I Do", how do I know how money downstrokes and upstrokes in what combination are needed for each chord(C,D,F...)?
Hmm .. it's a combination of ..
- How the song goes rhythmically
- How you choose to (or have been taught to) play the guitar
- What's comfortable for you
In the song you quote, it'd probably the usual thing to see 8-notes (count in with 1 .. 2 .. 3 .. 4 .. 1.. Look into my eyes... : 8 of those counts) to be a downstroke per count.
It's really about how fast you can move your arm. That's quite a slow song so you can afford to do downstrokes in time with the 8 beat. That also leaves you with the up-strokes available for filling in for the odd 16-beat bit too.
For something a bit quicker like .. err .. Come Up & Seem Me (Make me smile), you'd probably feel more comfortable strumming downstrokes in 4-beats (ie half as frequently as above) so that you can strum down - up - down - up easily and steadily.
If there are odd / uneven beats eg where there's an accent in the rhythm (a punch on the note perhaps ahead of time etc) then you have to figure out for yourself how to handle that. Usually an accent is best played as a downstroke, but sometimes if there are a few in quick succession then you have to use down and up strokes to get the speed.
Regarding how many .. eg 3 down 4 up .. well downstrokes are usually followed by upstrtokes as your arm returns, of course, but not always. A lot of rock on electric guitars is all downstroke, or alternatievly the intro to Smoke on the Water is played with all upstrokes (just sounds better that way).
Regards how many per each chord: That's a direct result of how long the song stays on each chord. For that you need to learn to count the timing and start with downstrokes with your count. At this point we're getting into the realms of how to learn a song, which is a different question.