As usual I have an opinion, but it's not different from StevieD's.
- What is Best practice calling external css? @import or <link>
Often when you see @import in the <head> it's an old trick to hide the CSS from Netscape7 or some ancient browser... because it did not understand @import at all. So far as I know, everyone supports it now, but like StevieD I only use <link>. There may be some advantage to @import for some particular CSS technique, but I don't know it.
- what's ideal number of css call per page?
Same as the amount of radiation we're supposed to get on the job: as little (low) as reasonably achievable (ALARA). You certainly do not want a separate sheet for typography, resets, colours, and box properties. To that end, I will include/merge any CSS that comes with any JS plugins I download (they often have their own, separate CSS files). Same goes for any CSS files generated by fontSquirrel and those types of font services.
what's css size should be.