By default it is not possible to approve or reject multiple items in a SharePoint workflow. Having a look on the internet several solutions can be found for that. In the following I am highlighting some of them – I mostly like the ones using Javascript Code.
- Sohels Blog: Nice Solution using JavaScript and a application page which is used for the approval dialogue, SharePoint 201 only
- Spjsblog: JavaScript only solution, also SharePoint 2010 only
- Bernardo Hoen: Like the first one: Serverside approach to achieve the same
- And even there is a Codeplex solution available which can be used to solve the request
