There are too many to provide any kind of definitive answer. You cannot make a determination of needs based solely on the size of your user base.
What is the difference in skills of the large population vs. the small one? Does one population have a greater percentage of experienced users than the other? Does one population need a different mix of hardware and/or software than the other? Is the larger population geographically dispersed?
Even though the most obvious difference would be one of scale, that may not be true unless one makes several assumptions and takes the actual user out of the equation completely.
Assumptions:
1. The same organization is involved in both scenarios.
2. All computers involved have essentially the same hardware.
3. All computers involved have essentially the same software mix.
4. The change in user population (upward or downward) doesn't necessitate a change in hardware or software needed.
If you want to keep the user in the equation to help determine the amount of help desk techs needed, you must also make the following assumption:
The skill classification of your user base (expressed as a percentage of the total) will remain constant regardless of whether the population increases or decreases (i.e. X percent of users are beginners needing help finding the on/off switch, X percent of users are veteran users who almost never need help, and the remainder fall somewhere in between).
If you make all these assumptions then the differences will be simply one of scale.
If you are talking two completely different industries, then everything goes out the windows as far as comparisons. It is entirely possible that an insurance company (whose users are probably primarily clerical) with 1000 users will need much more support than a software development firm (whose users can be expected to be much more technically adept) with 3000 users.
Hope this helps.
