Just a recap of those countries currently requiring high-vis vests:
Belgium - no fine for not having one, only for not wearing one, when required (50-1375_!)
Czech Republic - only commercially used vehicles including cars used on commercial business.
Portugal (only for vehicles registered in Portugal). There are different fines for not having one (60-300_) and for not wearing one, when required (120-600_).
Spain (not required for motorcycles) Fine 50_ maximum - will accept fluorescent braces as an alternative,
Austria (not required for motorcycles)
Italy - no fine for not having one, only for not wearing one, when required (36_ minimum)
Croatia - no fines
Romania (only for vehicles with a GVW of over 3.5 tonnes)
Slovakia - fine for not having one or not wearing one, when required, 50_ minimum.
Norway - only required for vehicles registered in Norway. Must be reachable from the driver's seat. No fines.
Germany - only for vehicles used commercially, i.e. including taxis, courier vehicles, etc.
Finland - applies to all pedestrians outside built up areas.
Reflective clothing in general is acceptable. No need for vests according to EN471. No fines.
Note that in most other cases, the vests must be marked according to EN471.
In general, only one is required for the driver so long as other vehicle occupants stay inside the car and the need to wear one normally applies only outside built-up areas. Strangely enough, several countries refer specifically to the driver only so that theoretically, if a passenger gets out of the car instead of the driver, he or she would not have to wear one (but I wouldn't rely on it).