Recently, I came across this article and wondered if there really is a definitive answer to the question of who invented the number line?
Not mentioned in the linked thread is Bombelli, who although not widely published until after cited “candidates” Stevin and Wallis, came before them per Bourbaki’s historical notes (my emphasis):
(...) up to the end of the Middle Ages [the] “ratios” of Euclid were customarily described as “numbers”, and the rules for calculating with integers were applied to them without any attempt to analyse the reasons for the success of these methods.
Nevertheless we see R. Bombelli, as early as the middle of the 16th century, expounding a point of view on this subject in his Algebra  (*), which is essentially correct (provided that the results of Book V of Euclid are assumed to be known); having realized that once the unit of length has been chosen there is a one-to-one correspondence between lengths and ratios of magnitudes, he defines the various algebraic operations on lengths (assuming of course that the unit has been fixed) and, representing numbers by lengths, obtains the geometrical definition of the field of real numbers (a point of view which is usually credited to Descartes) and thus gives his algebra a solid geometrical foundation (**).
(*) We are concerned here with Book IV of his Algebra, which remained unpublished until modern times; for our purposes it matters little whether or not the ideas of Bombelli on this subject were known to his contemporaries.
(**) (...) Bombelli, in the same context, gives with perfect clarity the purely formal definition (such as one would find in modern algebra) not only of negative numbers, but also of complex numbers.
Somebody did or several people did. We just don't know whom and its exceedingly unlikely we will ever know. What we can find out is the oldest example of a number line.
I suggest for this the Ishango Bone. Its a baboon fibula with vertical marks ranged in a row representing numbers. Thus the simplest example of a number line. Its been dated to 9,000 BC to 20,000 BC with some arguing for 44,000 BC. The discrepency in aging is due to the nature of volcanic dust that buried the discovery site.
Plus I'd say that the sign for the number one in many cultures from across Eurasia is the straight line. The number line referring to itself. One of the first examples of self-reference and right at the beginning of the story of number!