General
Affected tag(s) to be modified/added: allotments=plot, ref=*
Question asked: Is there an example tag on item XY?
https://taginfo.openstreetmap.org/tags/allotments=plot#overview
The quest will help adding plot numbers to allotment plots.
https://wiki.openstreetmap.org/wiki/Tag:allotments%3Dplot
Checklist
Ideas for implementation
I already forked and implemented it, because I chose to do it for a course at my highschool.
https://github.com/Hegik/StreetComplete/tree/allotmentsPlot
I still wanted to discuss it with you before making a pull request.
The quest is limited to countries part of the "Office International du Coin de Terre et des Jardins Familiaux" because they are organised similary.
I limited the plotnumber to 5 chars, so that absurd large allotments with 1000+ plots are supported and there is still room for a letter, to get all the edge cases.
This is how it looked, after I implemented it:

General
Affected tag(s) to be modified/added: allotments=plot, ref=*
Question asked: Is there an example tag on item XY?
https://taginfo.openstreetmap.org/tags/allotments=plot#overview
The quest will help adding plot numbers to allotment plots.
https://wiki.openstreetmap.org/wiki/Tag:allotments%3Dplot
Checklist
Ideas for implementation
I already forked and implemented it, because I chose to do it for a course at my highschool.
https://github.com/Hegik/StreetComplete/tree/allotmentsPlot
I still wanted to discuss it with you before making a pull request.
The quest is limited to countries part of the "Office International du Coin de Terre et des Jardins Familiaux" because they are organised similary.
I limited the plotnumber to 5 chars, so that absurd large allotments with 1000+ plots are supported and there is still room for a letter, to get all the edge cases.
This is how it looked, after I implemented it:
