How often should I notify referees about my PhD application? I will be applying now for PhD programmes and I wonder if I should notify my referees about every single application, providing data, such as: title and brief description of the project so they can tailor their references? Or that would be too much for them to get every single time an email like that? I know that they are busy with academic or own research commitments so I don't want to pile them with emails if that's not necessary. They are aware that I'm applying now.
My suggestion would be to get in touch with your referees and tell them you that you will be applying for several PhDs, so they should expect to potentially get multiple requests for references.

You can then tell them that if they receive a request for a reference and would like more information about the PhD, so that their can tailor their references, then you'll be very happy to provide that for them as necessary. It may of course be the case that your referees will not be inclined to tailor references to individual projects.

This way they're only receiving information when they would like it.