You can only support one charity with each fundraising page you create. However, you can support multiple charities by creating a new Fundraising Page for each charity.
Whilst it might seem straightforward to offer fundraisers the opportunity to support multiple charities with a single page, the complexities of doing so are significant - for example, apportioning Gift Aid from those sponsoring your activity and allowing fundraisers to decide the percentage of funds raised allocated to each charity. It is something we are frequently asked and it is on our development timeline, but it is not currently possible to support more than one charity per fundraising page.