Maximum number of external disks attached

Hello,

please what is the maximum external disks (SSD, SATA) attached to up squared? So far I succeded to get working only 3 disks accessible in Ubuntu at the same time:
1. mSata
2. single port USB 3.0
3. first of dual USB 3.0 ports
If I attach at the same time also to second port of USB 3.0 and/or USB 3.0 OTG - these disks are either not visible or not properly working.
I am using also 2xUSB 2.0 cable for Keyboard

Also utilizing mPCIe (mSata) and M2.2230 would be nice but I am expecting slower speed compared to USB3.0 (e.g. each disk is benchmarked for cca.500MB/s read).

Thanks Milos