

It basically needs critical mass.
The monsters were humans trying to engineer with thr molecule, that sample of protomolecule was not bound to become the ring.
On Eros, it was let loose specifically so that it could get enough mass to do what it was going to do. Think of it like a transistor- a few transistors can get you a NAND gate, but a LOT of them can give you a processor.
Once it had the critical mass and went to Venus, it was big enough that it could assimilate whatever it wanted into itself and gain the necessary mass to form the ring.
Fwice - 4x. Once, twice, thrice, fwice.