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Slices only applies to bread by itself.  A sandwich is usually made from two slices of bread.  "A sandwiches" would be wrong, since there are more than one.  You can say:

"three sandwiches"

"some sandwiches"

"the sandwiches"

"I brought two sandwiches to work with me."

"We will fix some sandwiches for the picnic."

"We need to make sure we bring the sandwiches when we come."

She used three slices of bread to make a club sandwich.

She made a sandwich for lunch.

Mother said, "I'll make you boys some sandwiches for lunch."

May told the counter clerk, "I'll take the sandwich home with me."

Dagwood exclaimed, "I've had many sandwiches in my day, but that was the best."

Only a few sandwiches were left.

Most sandwiches are eaten at lunch.

Sandwich is singular meaning one sandwich. Sandwiches is plural meaning more than one sandwich. The article "a" is used with the singular to indicate a sandwich in general. "A" is not used with the plural. The article "the" is used with the plural to indicate specific sandwiches. The plural sandwiches does not require an article -- "Mother made sandwiches for lunch."