why do sites need lots of servers?

It depends on how their back end if set up. Especially if they are running off DB's.

There is no simple answer, but a lot of time it has to do with load balance. However, typically those sites will have a ton of content. Like a tube huge site for example.

An example would be having the content on one server, and main site running of another, etc..

how do you know if your server is being over worked?

is it a case of it just crashing?

Again, it depends on the type of site it is and the script.

1. It could be loading very slow. Even after you do a trace route and see no issues.
2. You should be getting MySQL errors.
3. The script could be crashing out.
4. Videos taking a long time to load and stream.
5. Videos getting insanely choppy.
6. Your site just hangs.
7. You get a "too many connections" message. (in regards to DB)

Those are just some examples of video heavy sites. But there are plenty of others depending on the type of site.

Keep in mind, a lot will also depend on the SCRIPT and it's use of resources. Not all of it is because of servers, and lack of memory issues. Some scripts on the market are TERRIBLE in the way the use resources, and can be memory hogs.

Ask your host to check your servers load.

to speedup traffic bust

The other day I read that isohunt dot com has like 70 servers...

.