Each bus transaction has a request and a response, each taking 50 nsec, or 100 nsec per bus transaction. This gives 10 million bus transactions/sec. If each one is good for 4 bytes, the bus has to handle 40 MB/sec. The fact that these transactions may be sprayed over five I/O devices in round-robin fashion is irrelevant. A bus transaction takes 100 nsec, regardless of whether consecutive requests are to the same device or different devices, so the number of channels the DMA controller has does not matter. The bus does not know or care.