TC's are good for booming. We all know that but how many should you really have.

I believe the answer to that question all lies within the purpose of TCs.

-they produce villagers therefore
----more TC
----more villagers
----more resource build up

i did not say more resources since while you boom villagers its hard to hold up a military the uses food at the same time. At least till you get about 20-30 farmers going, and then you are set...

-the are garrisonable therefor
----under short knight raids they provide safety all at a
touch of the bell
----the are gathering points for villagers dropping off food

-they could be used to attack with therefor
----having a few with villagers garrisoned in enemy base could provide some damage to his town.

These are the purposes that i can think of right now.

NOw to answer the question.

If you strecth your self out of your base (meaning you see a gold pile a long way off) place a TC there to protect the villagers and to boom off villagers there. All the while your other villagers stay at home safe and continuing to gather resources.

Now what i do is simple.
This depends on the following.
-what strategy you are using
-what your enemy or enemys are doing

If i am in the pocket surronded by team mates, and if i have not been scouted i would boom with at least 4.

It is pointless to have more TC than you can boom villagers from unless you are attacking or protecting villagers with it.

Now if i am on the edge i would go 2-3 at home and then stretch out. Useally i am build villagers knights, and pikes all at once, and then trying to get a few black smith techs so it is very hard to anything else with more than 3 tcs.