http://aquacomputer.de/downloads/aquaero5_aquabus.pdf

3 or 4, you can connect both types.

Quoted from "Beercules"

I'm seeing conflicting comments on different sites to the limitations of the aquabus, and what connections are required (3pin or 4pin) cables. Can someone please clarify, how many AC pumps, MPS devices and Farbwerk or other Aquaero can connect to a single Aquaero (5LT, 6PRO or XT) and what connections are required.
Thanks in advance.
Beercules!

Be aware the 'low speed aquabus' port is disabled in aquaeros updated to the latest firmware, so we will discuss the 'high speed aquabus' only.

The quote below from Section 4.6 of the aquaero manual, expands on the aquabus. The point of interest is the pin assignments at the bottom... pin 4 is +5 volts and is only needed for devices that require power, i.e. mps flow sensor or mps pressure sensor. Devices such as the Aqua Computer D5 pump motor with USB and aquabus interface have a seperate power source and do not need the 4th pin of the aquabus, BUT, the 4th pin is not a hinderance, so you can use 4 pin aquabus cables exclusively and not see a problem. The converse is not true.

Quoted

4.6. Connector “aquabus low speed/high speed”
Connectors for communication with other devices from Aqua Computer. The
aquaero 5/6 features one „low speed“ (aquabus 1) and one „high speed“
(aquabus 2) port.
Products compatible to the “low speed” port:
● multiswitch USB
● tubemeter USB
Products compatible to the “high speed” port:
● aquastream XT
● Aqua Computer D5 pump motor with aquabus interface
● flow sensor „high flow USB“
● flow sensor mps flow
● poweradjust 2 USB (firmware version 1003 or higher)
● aqualis XT series
● aquainlet XT series
● pump adapter for aqualis with integrated fill level sensor
● pressure sensor mps pressure
Please note: Both aquabus connectors are also compatible with 3 pin
aquabus devices. The additional “pin 4” supplies power to compatible 4 pin
aquabus devices. For example, a poweradjust 2 unit (3 pin) and a mps flow
200 unit (4 pin) can simultaneously be connected using a 4 pin Y adapter
cable (art. 53124).
Pin assignment: Pin 1: GND
Pin 2: SDA
Pin 3: SCL
Pin 4: +5 V

As to the number of devices connectable to an aquaero, let me refer you to the table at section 25.1. Compatible aquabus devices of the manual. Each item connected to the aquabus requires a unique address. There are 4 addresses available (12, 13, 14, 15) to mps devices. The devices may all be the same or a mixture. Personally, I have two D5 pumps with USB/aquabus (12,13), one mps flow sensor(14) and one mps pressure (level) sensor (15).

I hope this helps. There is a lot of information in the aquaero manual.