The number of approval request emails received by a Contact is directly related to the number of candidates they have, and how often those candidates request approval.
If the contact only has a few candidates, instructing candidates to wait until the end of the period before requesting approval can be a simple solution to reducing the number of approval emails they receive.
What if my Contact has too many candidates to approve via email?
A possible solution is to invite Clients to have User Access to Invoxy (you can find more information on inviting users here: Inviting people to use Invoxy) so they can log in and approve any pending requests in bulk. You can see what their access to Invoxy would look like here: Contact User Access
You can also enable the 'Mute Approval Emails' setting on their Contact profiles in order to prevent approval emails from being sent to them.
Since those approvers won't be receiving approval emails it can be a good idea to use scheduled reminders as a way of reminding them to action their pending approvals. Learn more about schedule reminders here: Scheduled Reminder Messages
By going to Manage > Templates > Message Templates > Weekly Reminder - Approvers you can edit the existing template. Keep in mind that the same template is used for both approvers with and without the 'Mute Approval Emails' and/or User Access to Invoxy, so make sure the information contained in that message is still relevant for all approvers.