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As Leo has already noted, routers are really sized based on their capacity, not number of users. I've attached a Cisco whitepaper that explains capacity of various Cisco ISRs and provides recommendation based on "WAN" bandwidth.
To your later question, small office ISRs (i.e. 800 series) often provide some built-in L2 (Ethernet) edge ports. This so you don't have to also provide a separate L2 switch. Bigger L2 routers just provide ports for their usage, i.e. there's an assumption L2 edge ports will be provided by separate L2 switch(es). However. often the mid-size ISR can take optional L2 expansion cards, making them much like a all-in-one, but bigger, ISR 800 series.
