The Spurs don't have any cap room.
This is about the "apron" that is set 4M above the luxury tax threshold. Once a team uses more than the mini-MLE amount of the MLE, a de facto hard cap of 4M above the lux tax line is created.
That number figures to be about 74M. The Spurs have about 48M in current contracts. So "what's left over" is however much of that 26M that isn't used on the MLE, BAE, vet min contracts, and keeping Green and Mills.