I´m out in the countryside. Will it be impossible to find a pokemon here? Do you/should you always have a pokemon "in sight" when you zoom out, or will there be areas (eg. countryside) where there will be none, forever?
It depends on some server side logic. The way the game currently works is, every Pokemon is visible to every other player in that region. The Pokemon will have the same move's, size, weight and percentage level (The semi-circle behind the Pokemon in your inventory).
But there is one exception to this rule and that is incense. Once activated, you will see Pokemon occasionally spawning around you for up to 30 minute's that have a purple aura around them, just like your character, BUT other players can see them and they also don't appear in your Pokemon tracker.
Now with all that in mind, since there is 'x' amount of Pokemon spawning around any area at any time, there is a chance to collect all the Pokemon in that area even tho other players can see them since they have not collected those Pokemon themselves.
Lastly, when first logging in you might not see any Pokemon around you for a few minutes as the server did not need to create Pokemon in that area as no one was in it prior to you logging in, so you might need to move around a little bit before some Pokemon spawn.