Over my lunch break I extended my printer test program to make it easier to specify the width of the printing. It prints a sequence of numbers that make it easy to tell how many characters have been printed:
Everything works fine through 17 numeric characters (20 characters total, including the three symbol characters on the far right edge). When it attempts to print 18 numeric characters the last (left-most) two characters over-print. Attempting 19 numeric characters causes the three left-most characters to over-print. This doesn't seem to cause any mechanical problems, but I'm not planning on stress-testing this.
Clearly the limit for the Epson M-32TL printer is a total of 20 characters, composed of up to 17 numeric characters and three symbols on the right edge.