I am using:

PIII-550
512Mb RAM
SCSI HDrive
50Gb BW

Supposed I have a Perl script of 40Kb (which executes and searches a flat text data file of 1Mb) and people are accessing the script **at the same time***. Can someone give me a very rough estimation how many people it can support concurrently?

I am totally new to dedicated server and I need only a very rough estimation: 10? 100? 1000? 10,000?

Nobody has an idea?

If you have a server, use Apache Bench.

ab -n 1000 -c 10 http://yourpath/to/script.pl

And change the -c and -n until you kill the box.

As for answering the question, its a bit of 'how long is a piece of string'

That really depends on a lot of different things though.

What else is running on the machine etc.
What is going on in the background, logcheck amount other logs, backup scripts etc.

Since all of that varies from one moment to the next that isn't really a question you can answer.