I just bought a 7 port Belkin and it works great with 2 microphones. It has an external power source.
Belkin part# F4U018-BLK
and* Nintendo has not licensed any makes of commercially available USB hubs for use with the Wii console. Accordingly, Nintendo cannot guarantee that they will all be compatible with the Wii console. However, without wishing to make any particular recommendation, Nintendo can confirm that it has tested the Premium 4-Port USB hub manufactured by Logitech and that it appears to function properly.
source - USB Hub - Wii Support - Nintendo
Known Compatible USB Hubs
Nintendo has not licensed any makes of commercially-available USB hubs for use with the Wii console. Nintendo has tested the following products and found that they appear to function properly:
Belkin # F5U304
Belkin # F5U234
source - | Nintendo - Customer Service | Wii - Known Compatible USB Hubs