Couldn't find any definitive info. For different categories of vehicles and in various parts of the world?
Edit: I'm looking for the answer in exact US dollars. Or euros. Or pounds. Or whatever currencies.
It will depend on credit they are offering you during the period being covered. A gas station locks up what they expect is the maximum transaction for most people. When the prices of gas spikes some people have the pump turn off before the tank is filled, therefore they need to use a 2nd card to complete the purchase.
Before you arrive at a hotel they lockup the cost of one night in the hotel, that way they still sell the room for one night if you never show. While you are there they lockup the cost of what you could owe them. This would include the cost of the room, and average room service or bar service.
For a car rental, it would be based on the risk they perceive. They don't want to try and collect against a card you gave them when you reserved the car, or when you picked up the car, only to find that you have gone over the limit.
Some online systems will let you see what is pending against your card. Others could provide that information to you over the phone.
The amount can vary by car rental company and geographic location.
I did a Google search using the search term: "enterprise credit card hold"
The range went from $100 to $500. There is also mention of Jamaica for $1500.
The benefit of a credit card in this instance is that the hold is on a virtual balance versus a debit card that will actually hold the money that is in your checking account.