You could also check if there are parcel features without geometry (empty geometry) in your dataset. This will throw the same error.
ArcGIS Urban doesn't have a limit regarding the number of parcels you can upload. Just be aware that a huge number of parcels can reduce performance, especially inside plans.
For what I experienced, first, parcels must be in wkid:3857 without curves in the geometry. Also, there should not have identicals parcels overlapping and multiparts are suggested to be desagregated in single parts.