'Surely not!' Gladys replied, peering at the tea cloth. 'I can't see any mark!'
'But there is!' Mrs Oliphant persisted.
'Well, maybe,' Gladys conceded. 'But it's a very small drip!!!'
It should be clear from the dialogue if something is truly dramatic, rather than using the exclamation mark to spice it up like chilli powder sprinkled on yesterday's mashed potato. The conversation above is not dramatic, and no adding !!! will make it so. If anything, it draws attention to the weakness of the drama.
Sarah's Golden Rule: Three exclamation marks per novel is the maximum allowed.