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The number of entries is a member config decision; ten would be small.
Storing by entry number would require a mapping from entry number to position, or (trivial mapping) with all entries being maximal size. We pack them (saving space/increasing capacity) and reference by start byte number. The byte number needs more bits to encode than an entry number would, but the scratch ops are otherwise small and currently we just burn the entropy. The belt uses full sized entries and doesn’t try to pack because the actual belt is a set of latches/regs that must be full width anyway.
The choice for scratch implementation is left to the hardware guys, and might be different in different members due to hardware/cost/power considerations.