Gratulations (and thank you again!), looks like you/we finally made it 
!
For documentation’s sake:
working code:
var logger = Java.type('org.slf4j.LoggerFactory').getLogger('org.openhab.rule.' + ctx.ruleUID);
var Collectors = Java.type("java.util.stream.Collectors");
logger.info("Collectors imported, about to filter the Group members");
var numOnList = ir.getItem("Leuchten_Alle")
.members
.stream()
.filter(function(i){
logger.info("Processing Item " + i.name + " whose current state is " + i.state);
var currState = i.getStateAs(OnOffType.class);
logger.info(i.name +"'s state is " + currState);
return currState == ON;
})
.collect(Collectors.toList());
logger.info("The filter returned the following Items: " + numOnList.toString());
logger.info("There are " + numOnList.size() + " lights that are ON");
events.sendCommand("AnzahlLampenAn", numOnList.size());
Example of a result:
2021-04-16 17:36:34.472 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Collectors imported, about to filter the Group members
2021-04-16 17:36:34.473 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Processing Item EsszimmerTisch_Farbe whose current state is 29,83,0
2021-04-16 17:36:34.474 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - EsszimmerTisch_Farbe's state is OFF
2021-04-16 17:36:34.475 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Processing Item Frosch_Farbe whose current state is 46,56,0
2021-04-16 17:36:34.476 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Frosch_Farbe's state is OFF
2021-04-16 17:36:34.477 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Processing Item LeuchteHueklein_Farbe whose current state is 46,56,0
2021-04-16 17:36:34.477 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - LeuchteHueklein_Farbe's state is OFF
2021-04-16 17:36:34.478 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Processing Item LeuchteHuegross_Farbe whose current state is 36,30,21
2021-04-16 17:36:34.479 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - LeuchteHuegross_Farbe's state is ON
2021-04-16 17:36:34.480 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Processing Item LeuchteMobil_Farbe whose current state is 0,0,20
2021-04-16 17:36:34.481 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - LeuchteMobil_Farbe's state is ON
2021-04-16 17:36:34.492 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Processing Item EsszimmerWein_Farbe whose current state is 29,83,0
2021-04-16 17:36:34.493 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - EsszimmerWein_Farbe's state is OFF
2021-04-16 17:36:34.494 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Processing Item EsszimmerKlavier_Farbe whose current state is 29,83,0
2021-04-16 17:36:34.495 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - EsszimmerKlavier_Farbe's state is OFF
2021-04-16 17:36:34.496 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - Processing Item LeuchteWohnzimmer_Farbe whose current state is 29,83,0
2021-04-16 17:36:34.497 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - LeuchteWohnzimmer_Farbe's state is OFF
2021-04-16 17:36:34.498 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - The filter returned the following Items: [LeuchteHuegross_Farbe (Type=ColorItem, State=36,30,21, Label=Farbe, Category=ColorLight, Tags=[Lighting, Point], Groups=[LeuchteHuegross, Leuchten_Alle]), LeuchteMobil_Farbe (Type=ColorItem, State=0,0,20, Label=Farbe, Category=ColorLight, Tags=[Lighting, Point], Groups=[LeuchteMobil, Leuchten_Alle])]
2021-04-16 17:36:34.499 [INFO ] [org.openhab.rule.Leuchten_An_Anzahl ] - There are 2 lights that are ON
==> /var/log/openhab/events.log <==
2021-04-16 17:36:34.462 [INFO ] [openhab.event.ItemCommandEvent ] - Item 'LeuchteMobil_Farbe' received command 0,0,20
2021-04-16 17:36:34.466 [INFO ] [penhab.event.ItemStatePredictedEvent] - Item 'LeuchteMobil_Farbe' predicted to become 0,0,20
2021-04-16 17:36:34.469 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'LeuchteMobil_Farbe' changed from 0,0,0 to 0,0,20
2021-04-16 17:36:34.501 [INFO ] [openhab.event.ItemCommandEvent ] - Item 'AnzahlLampenAn' received command 2
2021-04-16 17:36:34.504 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'AnzahlLampenAn' changed from 1 to 2