You can use as many or as few as you want

9 will give you better security (you need to hack 9 accounts  to get into
each area of MOSS) - 1 will give you less security (hack that and you
get everything)

Very much you should have a separate app pool and separate account for
that app pool from the one you use to run the sites

I would also recommend that you have one to crawl external content that
has no permissions on moss  itself

That'll get you down to 3

If you want a separate one for the SQL services (highly recommended if
you have different databases on your sql server) then you can do that -
you might want another separate one for the SQL agent to send SQL alerts
to your SQL admins

Like I say, the bottom line is it entirely depends on your organisation
and how secure you want it to be / how much separation of roles you
require

If you're a small org with one person or department looking after
everything you can certainly use fewer accounts

For the test instance I have running (3 server configuration), I used one
account and gave it appropriate permissions to everything. That being said,
I'm still testing all the new goodies so who knows what it will end up being
...

I had separate accounts  for Search, Excel Calc Services, Internal and
external App Pools and DB's, a domain setup account, SSP internal /
External etc

I think I use 3 or 4 principle ones when I get lazy - I don't have that
information readily available