Only 1.
There are many Pokemon you catch that are similar in sapphire and emerald but there are many Pokemon emerald can only get and not sapphire.
The Red Tent is a decoration item in Pokemon Emerald for your Secret Base. There are many places around Hoenn that you can turn into a Secret Base. Note that most bases can only be caves or tree-houses.
385 Pokemon.
Pokemon Emerald has a bunch of Pokemon, that are available to be caught! It's too many to list here.
Alot
420
4.
Emerl? If you mean Emerald, there is 387 that you can get by trading (complete the data for every single Pokemon exept sinnoh Pokemon) but idk how many just for emerald.
8.
1
8
382