To share my experiences with you...
I dual-boxed originally to "carebear" level my toons. My lowbie warrior was being followed by my 70 holy priest. No downtime. You can level them up extremely fast like this. I saw this site, added a 3rd and 4th. Loved it. My wife didn't, so I had to give her the 4th account to keep her busy. She loves it now.
I find 2 boxing good for leveling and solo'ing PvE world content. Maybe grinding for mats, dailies and if you have some good friends on your realm, you can run 5-mans (or 10-25) and be effective. Having only 2 in PvP is more of a handycap than 1. You don't have enough burst damage to "one-shot" people. It may happen on some undergeared/underlevel people, but not often. You may be stuck healing each other for the duration of a fight and never put out any dps.
3 boxing... you can solo more PvE world content, and faster. PvP becomes a little more fun as you can now one/two shot most people. You can do well in most BG's because you still make up a small portion of the team. You can defend nodes/flags/etc by yourself... and do it pretty well while you still have enough people to do "other things". Depending on your group make up, you may need to only get a tank or healer. I've 4-manned reg. Magisters Terrace with a friend (warlock) and my 3x shammys.
4-5 boxing... solo even more PvE world content and instances even faster than 2-3 boxing. Completely self-sufficient. You can take out waves of people in PvP and you will be tough to take down. You may find WSG, AB and EOTS a little more difficult since you roll in 4/5's. This is half of the WSG team, meaning everyone else needs to do whatever your not doing and it is very hard to get people to let you be by yourself. If you're defending, they need to get flag. Same thing in AB/EOTS... Most people see a boxer and want to run w/ you (and ask stupid questions) so nothing gets done and you lose often. However, you can change the outcome of an AV by yourself. You (obviously) have the potential to do very well in arena.
I find that 3-boxing is a good balance, for me. I'm pretty social and I have a lot of RL friends that play the game. I find that I'm always grouping with at least 2-3 people a lot of the time. If I had my 4th, he would probably be sitting out most of the time (unless I'm pvping).
My advice to you... Box the most that your system can put up with because when you're 70, it's much easier to sit one out than to level another up.
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