According to the Spelman room draw times PDF, 232 students signed up for Spelman draw. The University offered 46 Spelman quads (184 spaces) - Spelman doubles were not part of this draw.
Because signing up for multiple draws is allowed, it is possible that some students do not utilize their Spelman draw times, choosing instead to draw into upperclass dormitories or residential colleges.
Assuming the first 184 people in independent draw all drew rooms in Spelman instead of dropping out, this would leave 48 students - 32 of them independent, according to the Spelman room draw PDF - without a room in Spelman. (This is the greatest possible number of independent students who could have been denied a room in Spelman under these circumstances.)
The Daily Princetonian Editorial Board was able to confirm that the third-to-last group was denied a room in Spelman, meaning that at least that group and the groups below it could not draw a room in Spelman. This adds up to least 12 students, six of them independent. (This would occur if nine quads' worth of people had dropped out of Spelman draw ahead of this third-to-last-group, and is the minimum number of students who did not get a chance to draw rooms in Spelman.)
This is how the Board arrived at its conclusion that between six and 32 independent students were denied the opportunity to draw into Spelman.