Implausibility only went up to 10 for me. Someone else left a comment on the Amalgamy-reducing option that for them, it only increased up to 12. It seems very likely that the limit depends on some variable. We just need to find out what it is. For the record, the skeleton on which I observed it was a segmented ribcage (3 segments), 8 antiquity and menace both, 8 wings and a tail, and 2900 pence of value at the time I ran up against the limit.