You are nowhere near the maximum for software distribution. I'm not positive that a maximum has ever been put on it. As an example, I was recently made aware of a company that distributes 20,000 (!) packages to close to 120,000 clients in several states and countries.
The only restrictions you may have are in your current hardware setup. If you could provide more information on your current hardware and network infrastructure, that could help in targeting whether you need an upgrade somewhere or not.
Remember that SMS 2003 is more reliable for software distribution (and everything else), particularly over slow or degraded links. So, if you are considering any kind of upgrade, it should be to the new version of SMS.
If you're looking for resources for SMS, look no further than Microsoft's site and myITforum.com. myITforum.com sees more visits than the Microsoft site due to the horde of information available and the kind and caring community there.
Microsoft's site: http://www.microsoft.com/smserver
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