Do IUPAC names for haloalkanes require numerals?
You must use the numbers unless your substance in a mixture of isomers; otherwise your molecules structure is ambiguous.  For chloropentane this name alone does not specify weather the chlorine is on the 1,2,or 3 position which is important as the position of a chlorine can dictated physical and chemical properties as well as reactivity in various organic reactions.  Same is true for trichloropentane.  there are there chlorine but without numbers I dont know if the chlorines are all on one carbon, if they are in the 2,3,4; 1,3,5; 1,2,5; 1,2,3; 2,3,5; 1,1,2; 1,3,3 1,1,4 positions or any of the many more possibilities.  all of these molecules give different products in various organic reactions.

Again though if is a mixture of isomers (as is typical of hexane solvents) it is not important to give number