"Are there any sandwiches left" Is correct because sandwiches are countable and plural. "Is there any milk left" -- milk is not countable, so we use "is"

The second is not quite correct. It it should be "Is there a sandwich left?" Both forms are common. The usual reason for asking a question like that is that you would like to take a sandwich. The second sentence suggests that if there is one sandwich left, you will take it. The first one suggests that if there are several sandwiches left, you will take one of them. The first sentence is slightly more polite, because you are only asking for a sandwich if there are plenty left.

The first asks if one or more sandwiches are left. The second asks if one is left. If there is one left, then there are also one or more left, so the two questions always have the same answer.

The second sentence is correct

Both sentences are grammatically correct, but the more commonly used form is "Is there any sandwich left?" When referring to an uncountable or mass noun like "sandwich," it's more common to use "any" with a singular form. In this case, "left" is the past participle of the verb "leave," and it doesn't change based on the number of sandwiches. Therefore, "Is there any sandwich left?" is the more natural and commonly used expression.