A. 1 | B. An infinite number | ||
C. 3 | D. A uncountably large but finite number | ||
He thought there was one. What appear to us as individual objects eg a Mr Whippy icecream are vortices in the 'plenum', a single 'stuff' which was how he thought of corporeal substance.
Strictly, he thought God was the only substance (since everything else was dependent on Him/Her.) But God aside there were two types of substance, mental and corporeal; lots of mental substances (minds) and just the one corporeal substance - according to Descartes.