They’re roads those cars usually doesnt pass. they’re along the neighborhoods, and mostly silent places. i asked this in an issue in streetcomplete github, they told me tag it as “no marked lanes” (osm quest was asking me how many lanes out there). this is okay. what about main streets, those havent marked lanes on them. should i count lanes by how many cars passing side-by-side or tag it as “no marked lanes” ? thanks for your ideas 
The number may vary. The ground truth could be the width of the road instead, to satisfy StreetComplete, together with a tag like lanes=unmarked.
should i count lanes by how many cars passing side-by-side or tag it as “no marked lanes” ?
I would do both, count the effective lanes and tag it as unmarked lanes
Have you checked the examples in the wiki ?
https://wiki.openstreetmap.org/wiki/Key:lanes#Examples
As long as the road is large enough for 2 cars to pass (4+ meters), lanes=2 + lane_markings=no works, otherwise lane_markings=no + width=X is the recommended approach for narrow ways.
More info here: When should the 'lanes=*' tag be applied?