Included: 4.272 currently recognized species of uncertain status, tagged as "provisionally accepted" - in average about 50% of them will later keep their status as accepted.
Included: 11.087 microspecies mostly in apomictic genera, especially Ranunculus auricomus agg. (Ranunculaceae) (941), Rubus fruticosus agg. (Rosaceae) (1.143), Alchemilla (Rosaceae) (678), Sorbus s. l. (Rosaceae) (206), Hieracium and Pilosella (Asteraceae) (5.397) and Taraxacum officinale agg. (Asteraceae) (2.373).
500 - 1.000 species are described each year and subsequently registered in IPNI (trend seems to decline!)
Therefore between 352.000 and 355.000 (including microspecies) is the correct number of validly described, non-fossil higher plant species as of today (2023), with about 500 - 1000 added each year. About +/- 10.000 tolerance exists based on differing species concepts (lumping vs. splitting).
Lugadha et al. (2016) arrive through "extrapolation" at a total of 390.000: clearly too high and nowhere within our "bottom-up", checklist-based numbers.
The total number of non-fossil, higher plant species - including the undescribed ones - will probably not exceed 400.000 by much.