How do I scale a terraform ec2 resource?

I currently am managing some EC2 instances.and an excerpt is

provider "aws" {}

resource "aws_instance" "worker_node" {

ami = "ami-asdfasdfagsdfgs"
instance_type = "m5.xlarge"
count = var.node_count

subnet_id = var.subnet_id
security_groups = [var.security_group]


this works fine for the initial setup but say I want to update the count from 3 to 5?
if I just update it and hit apply, it says it needs to destroy my instances and recreate due to the subnet and security group settings. I want it to just keep my running instances and add 2 more?