If I set the DX1000/DX2000 to High Speed mode, does the number of input channels decrease?

No, even in High Speed mode, the number of input channels is the same as Normal mode.
Note: In High Speed mode, the integral time of the A/D transformer is fixed at 600 Hz, and the power-line period noise rejection performance will be lowered. If measuring in an environment susceptible to power-line period noise, measure using Normal mode or the GX/GP dual interval measurement function.