[help]how many phone lines can be attached to asterisk

1)normally how many media gateways do asterisk support ; where asterisk acts as call agent
2)how many extensions can be attached to asterisk?
3)how many voice chennels or phone lines(64 kbps) can be attached and what is the appropriate hardware for interfacing to pstn?

You need to have a read of this book:

voip-info.org/wiki/view/Aste … +Telephony

Your questions are answered there.