That's one.
Two modems exist for the blue G3: Apple's and Global Village's. Panther supports both.
The Apple one is enclosed in a solid metal shell, and is better protected in the computer. The Global Village model (the one you found) is naked, but has Caller ID and (in Mac OS 9) GlobalFax support.
I bought the Global Village modem for my blue G3 and it's worked perfectly.
You might be able to find it cheaper on eBay.
tooki
P.S. Both models of modem are practically identical (they have identical layout, components, etc), and so both work correctly with the "Apple 56K Modem" modem script.