Where can I find a guide, tutorial, or calculator that tells me how
much bandwidth an ISP needs to provide users with a set bandwidth.  In
other words, if I am an ISP and want to advertise connection speeds of
128Kbps to my customers, how many people could I sign up to so
customers would on average receive the advertised speeds if my
connection was T-1?  How is this computed?

Hi chawimac!

Neither a tutorial nor special guide is required to figure out this
type of problem, only a little division (so a simple calculator,
however, might be useful.)

ISPs and large (usually telecommunications related) companies own and
lease high-capacity lines that serve as major backbones for internet
traffic.  Each type of high-speed line is designed and rated at a
particular maximum bandwitch capacity/ speed.  They often lease lines
to smaller ISPs which can be sub-leased as well.  You are speaking of
the last ISP on the line, so to speak, before the end user.  It also
sounds like you are describing a home or small-business user, with a
128kbps line.

In any case, to figure the appropriate number of users that an ISP can
lease it's bandwidth to, you must know two things: the total bandwidth
available (the SUM of all of their connections) as well as the
advertised speed alloted to the end user.  To figure the number of
users, you would divide (using the same units) the total available
bandwidth by the bandwidth allocated to each user.

For example, using your figure, say a small ISP is sub-leasing a T1
line to DSL subscribers, and they advertise that their clients will
have 128kbps speeds. A T1 line is rated at 1.544 megabits per second. 
This is equal to 1544.0Kbps.  So we divide:

[1544kbps / 128kbps] = 12.06, which rounded down would be equal to
bandwith available for 12 128kbps users.

I said that the first number must be the SUM of the ISPs lines,
because often an ISP will have a number of large lines that they use
as backbone connectors (for example, 6 OC3 lines, or something of that
nature).

The example I gave was rather small-scale, but should effectivly
demonstrate the way these numbers are calculated.  Large ISPs, in
reality, need gigabits worth of available bandwidth, and have hundreds
of thousands of users.  They use many different types of leased lines-
a fairly complete listing of line speeds is available at:
[ http://www.t1-t3-internet.com/broadband+speed.html ]

The amount of money the larger lines cost can be staggering (some
backbone lines in excess of $100,000 a month); some are listed on the
perviously linked page.

Also, ISPs could allow more users on their networks than they really
have support for, which probably happens quite frequently, except as
long as everyone doesnt start heavily using their connections at once,
it doesn't cause too many problems.

I hope this satisfactorily answered your question.  Please let me know
if there is anything else I can help with.  Thank you for using Google
Answers!

Regards,
Andrewxmp

Search terms used:
line speeds T1 T3 OC3