Can we get some more info on the type of traffic you're trying to simulate? Whilst it's possible to simulate multiple IPs on one device, it may be easier to stress test web applications in other ways depending on the outcome you're looking for.
If your web server is on your current computer or local, you can assign multiple IP addresses to the network interface using the network settings page.
If you're trying to simulate website traffic, you can use something like https://loader.io/ or https://loadster.app/. They're both pretty respected and aren't random skid booters, they're designed to legitimately stress test. I'd just check with your provider first and see if you need to let them know before conducting any stress testing, as you're ultimately testing their resources. Most already have clauses in their AUPs addressing it.