It’s 64MB per project and 1GB per device. The samples are mono so you really have to work hard to fill this 64MB space.
I would guess that most of the Digitakt users have never even seen that. You would need literally hundreds of samples to fill the RAM as most of the samples will be small snippets anyway. This is one of the nice features of Digitakt - it is a flexible sound design device with a small learning curve. It encourages you to think small and teaches you to get the most from the sound you have. Here is one example what you can do with only one single (!) noise sample: https://www.elektronauts.com/t/made-this-song-with-digitakt-using-no-external-samples