If you use the $read identifier with no switches, it will read a random line from your file.

Replace <path\file.txt> with your own path and file name.

Code:
on 1:JOIN:#: {
 .timer 1 10 msg $nick Welcome
 .timer 1 20 msg $nick $read(<path\file.txt>)
 .timer 1 30 msg $nick Thanks for visiting and enjoy your stay
}


If you meant you want 10 responses to choose randomly for each message, you could create a file for the welcome messages, a file for the info messages and a file for the thank you messages...

Code:
on 1:JOIN:#: {
 .timer 1 10 msg $nick $read(<path\welcome.txt>)
 .timer 1 20 msg $nick $read(<path\info.txt>)
 .timer 1 30 msg $nick $read(<path\thanks.txt>)
}