It is equivalent to Euclid's parallel postulate and was named after the Scottish mathematician John Playfair. It is only required to state "at most" because the rest of the postulates will imply that there is exactly one. It could perfectly be assumed to write it saying "there is one and only one parallel".

@vrusha In geometry, the parallel postulate, also called Euclid's fifth postulate because it is the fifth postulate in Euclid's Elements, is a distinctive axiom in Euclidean geometry. It states that, in two dimensional geometry: If a line segment intersects two straight lines forming two interior angles on the same side that sum to less than two right angles, then the two lines, if extended indefinitely, meet on that side on which the angles sum to less than two right angles.

Так так оно и есть же.