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2000 Ford Ranger - how bad is the alternator damage?

I accidentally tried hooing up the negative to the positive post and the positive to the negative post…what do you think the damage is at my alternater

Destroyed plus possibly several other electrical items. You will need to have it towed to shop and let a professional look at it . With any luck you may have just blown a fuse or more.

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If the key was off you might be lucky and get out for few $$$. But some close inspection needs to be made because fuses and the alternator are often damaged when the wires are crossed.

why don’t you start the car and start driving it around the block. If there is a problem it would be visible immediately.

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Were you installing the battery? Or were you attempting a jump start?

In any event, the fusible links are what your shop should check first. It’s possible if anything got damaged at all, its only the fusible links. These are inexpensive to replace. The engineers who design the car expect this will happen from time to time, and test that the electrical system design is fault tolerant for this eventuality. I’m not sure which would cause the most damage, installing the battery reversed, or reversing the polarity on a jump start. I’d guess the worst case would be the jump start.