It is common in electronic circuits to keep the input impedance high so as to draw little current from the proceeding stage but to keep the output drive capability much higher.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Digital logic "fan-out".
In digital logic, for example, on the older TTL chips the "fan-out" was often specified and told the designer how many inputs the output could drive. This was quite low - maybe 10 to 15, if I remember correctly. In Figure 1 the fan-out is 3 so no problems would be expected.
The ATMega devices are designed as general purpose MCUs and one of their selling points is versatility. They are not always used in simple logic. With their very low-current inputs they can monitor inputs with a high output impedance while with their 20 mA drive capability they can drive LEDs, etc, directly. Both features cut down the component count, complexity and cost on the finished design.
That probably does not mean that the output current of the MCU is 1 uA as well if directly connecting the two pins together (i.e. MCU output pin to ads114s08b input pin). Does it?
The output current will be determined by the input of the following device. The output will supply as much as demanded until the current limit is reached. If the following device requires 1 µA then that's all the output will have to provide.
In case I am right, what will the current be and how can I bring it to < 20 mA no matter what ?
There is no problem.
