If the Erupters are drawing 510ma
https://bitcointalk.org/index.php?topic=212085.0

And the Raspberry Pi draws 515ma under load as stated here:
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=6050&start=50

Than this is a rough calculation of how many amps you would pull in just powering Erupters:
Qty   Total Amps
1   0.51
2   1.02
3   1.53
4   2.04
5   2.55
6   3.06
7   3.57
8   4.08
9   4.59
10   5.1
   
And this is a this is a rough calculation of how many amps you would pull in powering Erupters and powering a Rpi off the usb hub also:
   
1   1.025
2   1.535
3   2.045
4   2.555
5   3.065
6   3.575
7   4.085
8   4.595
9   5.105
10   5.615