Hi Francesco,
DMA is configured in Descriptor mode for Ping-Pong (while one is getting filled by ACM/SPORT, other can be used for processing); therefore have two buffers. As per your requirement, you can have single buffer and configure DMA in Stop mode or Auto-buffer mode.
Regarding second query, in the current code SPORT DMA will continuously receive the data for enabled ACM events in the same order in which ACM events are handled. I used DMA mode because of higher sampling rate (to avoid core overhead).
But if you have events sampling multiple ADC channels, then you need to use core mode of ACM (instead of DMA based). In the SPORT ISR, you need to check ACM Event completion status register and place the received data in required buffer. That you need to handle in software as per your requirement.
Let me know if you need more details.
--Prashant