Image: unknown
The origin of the name is uncertain, but almost certainly dates from the 1930's when Cadbury began providing short-length flake chocolates specifically for ice creams, and Askeys, the cone-makers, started providing a 'dripless' cup cone, both, seemingly independently using the '99' branding. However, the earliest ice-cream treats sold as '99' were wafer sandwiches with a stick of chocolate in the centre (OED).
Possible reasons suggested for the name include:
●Ice cream is 'IC' which is one way of writing 99 in Roman numerals.
●Italy had, or a king had, or the Pope has, a group of 99 elite soldiers, so anything first-rate was dubbed a '99' by Italians, including those in the English ice-cream trade.
●The flakes are 99mm long
●A chocolate flake has 99 layers.
●They were invented by Edinburgh ice cream maker Rudi Arcari in the 1920s at his shop, 99 Portobello High St.
●They originally cost 6/6 per box of 99 items
99 Ice Cream
Image: 'distillated'