How many is too many

  I am looking at a table in our system and it has 12 indexes, we are planning on adding another one. I am aware of the effects of having too many indexes, but in this case adding an extra index helps a certain query that runs slow. Other queries and most other operations against the table are acceptable too. I see a few tables like this; is there a number above which is considered a no-no when it comes to adding more indexes.

PS. The tables and queries are structured in a way that seem to require several indexes - it is a third party product.

TIA,
Ram.

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