Playing a little with the Weland dahlia list (dahliasuppliers.com/biglist/biglista.htm).
I count 50.467 dahlias in the list (including a few species). It seems rather outdated: one cultivar is from 2009, but that is the only cultivar in the list after 2003. Also there are lots of typo's: 256 are from year 0, one is from year 29000, one from year 71971 and so on. Ignoring these errors, the oldest registered cv seems to be Florence Louise from 1804. The years with the most cultivars released are 1969 with 2225 CVs and 1958 with 2040 CVs. Bruidegom company in the Netherlands released the most: 1166. Roughly half of the CVs have a uncertain year of introduction: 23712
Looking at size:
Giant 1852
Large 6722
Medium 10781
Miniature 4423
Small 8254
(Blank) 18435
Color (roughly):
Light Blend 7736
Red 5916
Yellow 5495
Pink 3430
White 3361
Lavender 2610
Purple 2606
Orange 2545
Dark Pink 2443
Dark Red 2440
Bronze 2297
Dark Blend 2293
Bicolor 1615
Flame Blend 1257
Variegated 1043
I count 50.467 dahlias in the list (including a few species). It seems rather outdated: one cultivar is from 2009, but that is the only cultivar in the list after 2003. Also there are lots of typo's: 256 are from year 0, one is from year 29000, one from year 71971 and so on. Ignoring these errors, the oldest registered cv seems to be Florence Louise from 1804. The years with the most cultivars released are 1969 with 2225 CVs and 1958 with 2040 CVs. Bruidegom company in the Netherlands released the most: 1166. Roughly half of the CVs have a uncertain year of introduction: 23712
Looking at size:
Giant 1852
Large 6722
Medium 10781
Miniature 4423
Small 8254
(Blank) 18435
Color (roughly):
Light Blend 7736
Red 5916
Yellow 5495
Pink 3430
White 3361
Lavender 2610
Purple 2606
Orange 2545
Dark Pink 2443
Dark Red 2440
Bronze 2297
Dark Blend 2293
Bicolor 1615
Flame Blend 1257
Variegated 1043