The practical limit of each page of a Note Pad in Note Pad version 7.5 is 32,700 characters. Note Pad 7.5 is included with Mac OS 7.5 and later.
Earlier versions of Note Pad are limited to 8 pages total.
For information on working around these limitations, please see Knowledge Base article:
Article 25127: "
Mac OS: Using Multiple Note Pads and Scrapbooks"