How much does Apple Care+ cover for a lost iPhone

Apple has a list of service fees on their US AppleCare page here. This says the fee for a claim after theft or loss is US$149

You may also have received some documentation when you commenced your AppleCare+ plan. If so, refer to that.

Be aware too of the conditions outlined in footnote 5:

Theft and loss coverage requires you to have Find My enabled on your device at the time it is lost or stolen. Find My should remain enabled and your iPhone should remain associated with your Apple ID throughout the theft or loss claim process. During the theft or loss claims process, you will be asked to erase your missing device, disable it, and transfer ownership before you can be issued a new device. 

In addition to the great information David McKinlay provided, you need to know the difference between AppleCare+ and AppleCare+ Theft & Loss. You would have needed to pay for the higher level Theft & Loss coverage to make a claim if you phone is lost or stolen.

AppleCare plus doesn’t cover lost or stolen devices at all.

You have to purchase AppleCare plus with theft and loss coverage.

they are 2 separate plans.

AppleCare+ with Theft and Loss

Get everything included in AppleCare+ and up to two incidents of theft or loss coverage every 12 months.3 Each incident is subject to the deductible listed below. 

Theft and loss coverage requires you to have Find My enabled on your device at the time it is lost or stolen and throughout the claims process.5

AppleCare Products - iPhone - Apple