Depending on what you want to play, you'll likely want to use Recruit a Friend (RAF).

A lot of people play five characters, that is likely the average.
It is a full group, for anything PvE related and is a full group for Arena too.
You can queue as a group of five, for battlegrounds and have everyone get into the same game.

Four is enough DPS to generally blow people up.
It is also four of ten players, in the smaller battlegrounds which is an advantage over being five (in one place) of ten players.

Three is less than I'd like, but recently several people have been three-boxing.
It is the correct number for 3s in Arena, which has a lot more people queuing than for 5s.

Ten is a jump up from five, and is a common number as well.
You can attempt 10-man raids; Mercurio was quite successful doing them during the WotLK era.
With 10, you can be quite a force in world or battleground pvp.

Invisahealz is playing with twenty five.
Sam Deathwalker is playing with thirty two.
Prepared is playing with forty or fifty.
There isn't really a fixed limit on what you can do; it's more a question of your hardware and budget.



If you opt for five...
And have a starting account.

I would suggest two RAF chains.
A (Parent Account) RAFs B which RAFs C.
A (Parent Account) RAFs D which RAFs E.

You go into the account management page, and send an RAF invite to a friend, using your own email address.
Do this twice, for A > B and A > D.

Log into the page for B and send yourself an invite B > C.
Same deal for D > E.

Use digital codes, retail codes, or upgrade the accounts from the Battle.Net page for the account.
With eight or fewer accounts, having everyone on the same B.Net email is a good idea; one authenticator for every account and easier account management.
If you might go over eight accounts, it might be an idea to have two B.Nets from the start.
In general one B.Net is good.

With the two chains...
Your booster on A, boosts characters on BCDE.
Even though A is too high for the A>B and A>D chains, to count for RAF... the B > C and D > E links are both valid so BCDE all get the RAF benefits.
And because B & D are linked to A, they can give the account benefits to A.

For every four toons you boost on BCDE, you end up with:
A) Six characters, via Granted Levels.
B/D) Four boosted characters, plus two characters Granted.
C/E) Four boosted characters.

With seven characters on A already, you might be better off with five new accounts.
You can transfer any character, from the existing account... to one of the new accounts, if they're a favorite.
If the original account is on the same B.Net, you'll retain most mounts/achievements.
It is an option...