gistfile1.txt

variable "foo" {
    default = ["1", "2", "3"]
}

variable "bar" {
    default = ["a", "b", "c"]
}

resource "aws_sns_topic" "test" {
    name = "${element(var.foo, count.index)}-${element(var.bar, count.index)}"
    count =  "${length(var.foo) * length(var.bar)}"

}

gistfile2.txt

+ aws_sns_topic.test.0
    arn:    "<computed>"
    name:   "1-a"
    policy: "<computed>"

+ aws_sns_topic.test.1
    arn:    "<computed>"
    name:   "2-b"
    policy: "<computed>"

+ aws_sns_topic.test.2
    arn:    "<computed>"
    name:   "3-c"
    policy: "<computed>"

+ aws_sns_topic.test.3
    arn:    "<computed>"
    name:   "1-a"
    policy: "<computed>"

+ aws_sns_topic.test.4
    arn:    "<computed>"
    name:   "2-b"
    policy: "<computed>"

+ aws_sns_topic.test.5
    arn:    "<computed>"
    name:   "3-c"
    policy: "<computed>"

+ aws_sns_topic.test.6
    arn:    "<computed>"
    name:   "1-a"
    policy: "<computed>"

+ aws_sns_topic.test.7
    arn:    "<computed>"
    name:   "2-b"
    policy: "<computed>"

+ aws_sns_topic.test.8
    arn:    "<computed>"
    name:   "3-c"
    policy: "<computed>"