How many mmcblk devices is normal?
There is nothing abnormal in this case. They is only one device in the first preformatted text, which is /dev/block/mmcblk0 (just like /dev/sda in Linux distros). The rest are the partitions within that device (akin to /dev/sda1, /dev/sda2 and so on).

There is no normal limit, not that I came across, for how many partitions a device can have. It depends upon how developers want to design and provide a functionality. E.g. earlier, there used to be one system and boot partition. But recent Android versions started a new normal of setting up additional boot and system partitions for fail-safe measures. 

Also, mount command only shows the partition which are mounted under /. Some partitions are never needed to be mounted, such as ones concerning bootloader, so you won't see them in mount's output.