What is the number of channels that can be recorded at a scan interval of 100 ms with SMARTDAC+ GX/GP series?

It depends on the mode.
In 10 ch mode, measurement can occur on CH1-CH10; in 2 ch mode, CH1-CH2.
In 10 ch mode, A/D integral time is fixed at 1.67, and measured values (especially with temperature measurements from thermocouples) may fluctuate because power-line period noise is not eliminated. In 2 ch mode, A/D integral time is fixed at 16.67, and power-line period noise can be eliminated.

For operation mode settings, see the user's manual (IM 04L51B01-01EN)
◊1.7.3 "Setting the Operation Mode for a Module."
User's Manual (IM 04L51B01-01EN)