Car head Unit Accessory wire

There is no arbitrary limit to number of devices, just the total amount of power available.

Check the fuse supplying that line.

Then add up the power demand of each of the devices you plan to add. As long as the sum of demand is less than the limit allowed by the fuse, you're fine.

 

You'll have to research each of your devices. It'll be in the specs or instructions for each. Stereo may take 5 amps... maybe the processor takes 1, dashcam 1.5 etc... so add that all up. Then find the fuse feeding this branch circuit (owners manual ought to have this info) and make certain that your aggregate demand (7.5 amps in my example so far) is lower than the fuse rating. It is probably a 10 or 15 amp fuse (just guessing) so in this example you would be fine.

If you can't find the info, you can take direct measurements with an ammeter.

If your total demand is higher than the rating, you will have to power some of these devices from a different branch circuit. Never replace a low-rated fuse with a higher-rated fuse, that's a serious fire risk.

 

I would find another circuit to host the Alpine & lighting.

You are already slightly over the limit, but the head unit consumption is extremely variable, and will only have brief peaks at full rating. This is why the fuse hasn't blown already, despite that unit having a high demand listing. I don't think you need to take anything off that circuit, but I wouldn't add any more.

Find another switched accessory circuit, or create a new one with a new fuse feeding a new wire run from the underhood electric panel.