The first root node slot in the bps trees stores the number of used slots, not considering the first (or the data proper used slots). The number is stored as an unsigned integer of size matching the pointer size, in the first available slot bytes. The used slots follows contiguously the first slot.
For stability considerations the number of slots in the root node of the bps trees does not match the number of slots in non root nodes (and it is greater for the former).