why do we need to declare the maximum number of elements the index can store beforehand?

[siddhsql]

from the docs:

# Initializing index - the maximum number of elements should be known beforehand
p.init_index(max_elements = num_elements, ef_construction = 200, M = 16)

why do we need to declare the maximum number of elements the index can store beforehand? i don't see any constraint like this in the algorithm described in the hnswlib paper. thanks.