The PS100 (and CH100) are limited to 1.2 A of input. Because the SP20 20 W solar panel generates 1.17 A, that single panel represents a practical limitation to the input. For panels that generate current greater than 1.2 A, consider the PS200, Smart Power Supply and Charge Controller, or CH200, Smart 12 V Charging Regulator, for up to 3 A of input, or pn 18529, 10 A, 12 V, Morningstar Regulator, which is capable of accepting 10 A of input.
Generally, it is not recommended to connect more than one panel at a time. Differential shading can cause issues, such as the shaded panel acting as a current sink.