Latin (and ancient Greek) didn't have dedicated symbols for numbers, but at sone point the Romans started using a shorthand system based on existing alphabetic

Latin (and ancient Greek) didn't have dedicated symbols for numbers, but at sone point the Romans started using a shorthand system based on existing alphabetic symbols, which we now call "Roman numerals". It's quite possible the "I"s started out as just a line to mark one item before the shorthand was developed, but certainly over time it became identified to capital "I".