You could use a trunk, looping the call several times. For example, set up a DID routed to an IVR with an option that (via a Misc Destination) calls out to the same number. You can then enter that option as many times as desired for the number of calls.
I don’t know whether a loopback trunk will provide a sufficiently accurate test, though you could try that first then confirm your results with a real PSTN trunk.
Note that there are many factors affecting PBX performance. By far, the biggest one is transcoding, especially with Opus. Call recording comes next, followed by encryption and listening for inband DTMF.
Also, depending on your endpoints, trunks, routers, etc., it may be possible to set up Asterisk for Direct Media, so the (S)RTP passes directly between device and trunk and the Asterisk load is zero, whether or not encryption is used. Of course, this also requires that Asterisk not transcode, record the call, or listen for DTMF.