I think how much you can get on depends on the size of the dog, as littlies can't wear a tag big enough for more than the legal requirements.
Mine are both big dogs, ddog 1 has a slider tag that is securely fixed to the leather of his collar and includes:
His name + our surname
house number, street name, postcode
landline
mobile phone number 1 (mine)
moblile phone number 2 (dh's)
microchipped (UK)
He then has a traditional type hanging tag, with:
his name + our surname
house number, street name, postcode
landline (I figure if he got lost at least we'd get an answer-phone message that way)
Vet: vet's phone number
Microchipped (UK)
Ddog2 just has a double sided hanging tag that includes:
Front:
his name + our surname
house number, street name, postcode
landline
microchipped (UK)
Back:
mobile phone number 1 (mine)
moblile phone number 2 (dh's)
Vet: vet's phone number
He will be getting a slider tag too when I change his collar shortly.
The reason I like to do both is having worked in a rescue centre, we used to get quite a few dogs in that had the split ring, but no tag attached to their collar, so the slider tag is back-up just in case they snag the hanging tag and it gets lost. Sighthounds can cover a lot of ground and run through all sorts of hedges and ditches, so I prefer a belt and braces approach to their id tags. Also, most people would look for a hanging tag first and may miss a slider tag under their thick fur.
I always try and fit the street name on if I can too, as when our old girl got out of the garden - typically the one time she was collarless after having a neck operation - she was found in the park immediately behind our house, but they didn't know where she lived, so took her home and called the dog warden. I always think, if she'd had her collar on she'd have been home in minutes had it said both the house number and street name.