Why is a LIN cluster limited to 16 nodes?
If a certain allowed resistance and capacitance is specified for each slave node, and 15 slaves is allowed for total resistance and capacitance on bus for error-free operation with good margin, then adding yet another node can exceed the total allowed resistance and capacitance on bus and then it fails to operate within specification as there is too much resistive and capacitive load.

On the other hand, if all slaves load the bus as little as possible, it might work fine with one more slave, or ten more, but that's why there are specifications. If the whole system and each device on it is made by the specification, it is safe to assume it works. Having an ambiguous specification which says you can have as many devices as long as the total bus load, rise times etc are not exceeded, it would be difficult to calculate beforehand how many devices you can have there, and it would need to be measured if the electrical parameters are within specification and good margins.