I assume you just mean mobile data in general and, yeah, a few. My car has a remote app that works over WiFi, so I installed a small LTE router running OpenWRT with a WireGuard tunnel back to the home network that makes it accessible to Home Assistant anywhere. Now Home Assistant can keep an eye on charge/fuel levels, door locks and more, as well as remotely trigger the heater or start/stop charging. The SIM is £2.50/mo for 2GB which is far more than I need.

For other things like weather stations I'd be inclined to say that you'd be better using a less centralised, low-power network like LoRA (eg. Meshtastic).

Not sure about what you mean....

But here is an interesting talk about playing with sim cards:

https://youtu.be/WMII5G98AdM

My RV Home Assistant (running on a RPi5) could use one to stay connected all the time.

Problem is I don't want to pay the fees to do that.