Have had a few
Definition 1 of 2
Pro Tip 1/3Informal Usage
Used casually among friends, not in professional or formal settings. 
At the barbecue, John said he wouldn't mind if he had a few.
Pro Tip 2/3Understated Count
Might suggest modesty; actual number of drinks may be more. 
She laughed and admitted to have had a few when she tripped.
Pro Tip 3/3Ambiguous Quantity
The phrase doesn't specify the exact number, it could vary greatly. 
He casually mentioned he'd have had a few, avoiding details.