Does anyone run hella horns or any other horns?

is it fairly straight forward in terms of installing a new dual horn setup like hella horns

Not that hard, use the old horn wiring to trigger a relay that sends power directly from the battery to the new horns. Make sure to use thick enough wiring and an inline fuse, this way they're as loud as possible. A million diagrams out there just by Googling "horn relay wiring diagram" or something to that effect. Or if you're just doing it for the sound and not so much as a safety upgrade you can just put the new horns on the factory wiring and make life easier.

I actually have installed Lexus LS400/430 horns on some of my other 'yotas. Plug 'n' Play, and the sound is substantially better than that disk style buzzers used in the Celicas from the factory.

Get 'The Horn', you'll need to do a little more work like make a relay circuit triggered by a relay, but lemme tell ya it'll get attention like no other.
https://www.ebay.com/itm/150db-Truck-Mega-T...aha&vxp=mtr

It's loud, it's deep, it's awesome.

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Just curious how and what does a relay do

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does our cars come with one or two horns?

Gosh I messed up that sentence. A relay circuit triggered by the factory horn. Basically a relay is an electric switch that takes an input (the horn wire) and uses that to close a set of contacts to turn on the bigger horn. This protects the factory circuit from being damaged by the heavier load of the bigger horn.
http://www.explainthatstuff.com/howrelayswork.html
Plenty on the internet about how to use them.