Don't get more than 100 copies the first time out unless you have an unusually large family. Most agencies or production companies aren't going to want more than one tape. Agents, however are going to want between 5 and 15, but there aren't that many agents who really understand the voice-over market. Philadelphia, as an example, has virtually no agents, so everybody there is on their own. Where you are will determine how many copies you need.
Cost is the main factor.
You may think you're saving money, but you'll be buying commercial cassette blanks and, if they're decent quality, they cost a lot. They're also too long and you'll be wasting tape. Duplication houses and studios order the shorter tapes from professional supply houses. They cost less. Then there is the quality issue.