There is no universal convention. Most LAN routers and switches are four digit model numbers. The first two digits are the product series (e.g., 3800 series routers). For chassis-based Catalyst switches, the last two numbers denote the number of slots (e.g., 6509 nine slot chassis). That convention has been used on some routers as well (e.g., 7513 router).
If you really want to keep track of what the capbilities of a given product are at a glance, I recommend getting a copy of the Cisco Products Quick Reference Guide (QRG). See http://www.cisco.com/go/qrg for more information.
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