Using more than one index per table is dangerous?

Question

In a former company I worked at, the rule of thumb was that a table should have no more than one index (allowing the odd exception, and certain parent-tables holding references

Answer 1

Optimizing the retrieval with indexes must be carefully designed to reflect actual query patterns. Surely, for a table with Primary Key, you will have at least one clustered index (that's how data is actually stored), then any additional indexes are taking advantage of the layout of the data (clustered index).
After analyzing queries that execute against the table, you want to design an index(s) that cover them. That may mean building one or more indexes but that heavily depends on the queries themselves. That decision cannot be made just by looking at column statistics only.
For tables where it's mostly inserts, i.e. ETL tables or something, then you should not create Primary Keys, or actually drop indexes and re-create them if data changes too quickly or drop/recreated entirely. I personally would be scared to step into an environment that has a hard-coded rule of indexes per table ratio.