Jim,
I think Gen. Grant knew how many men he would have needed for he mentions in his memoirs that he would have rather tried to take Ft. Henry and Donelson at that time, then to wait a few weeks when the Confederates had many more men there. Also, he was receiving any reinforcements and supplies he needed during the seige of Ft. Donelson from Cairo, Mound City, and Gen. Sherman who was at Smithland, KY.

Had Ft. Donelson not fell when it did, he was prepared to have tents/shelters sent for his men who were up until that time perched outdoors on the hill with no shelter, heat, etc. in the freezing rain and sleet.

Perhaps the reason Gen. Johnston didn't send more troops was because Gen. Pillow had telegraphed him stating that his troops had had great success that day. Gen. Johnston forwarded the telegram to Richmond. While it was being read there, Pillow and Gen. Floyd were making their escape.

Had Gen. Buckner been in charge by himself, things would likely have been much different.
My thoughts,
Ann