Can a voucher be used more than once?

Yes, you can set up your voucher campaign to allow voucher codes to be used only once or for multiple times. You can specify how many times the multiple uses is for. 

Can I see which voucher codes have been used?

Yes, if you navigate to your voucher settings page and select "Codes" from the actions dropdown you will see which codes have been used and additional details about those purchases. 

I don't see where to create or manage vouchers. Where do I see that?

Under Products > Settings > Vouchers is the management of vouchers. Not all users have access to this feature and not all shops have this feature enabled. If you wish to utilize this feature and do not already have access please open a support ticket