Newbie Plugin question

Hello guys,
newbie inpatient question about pluggins.

if I purchase a premium plugin, can that plugin be used more than once?

is the fee for 1 plugin for = 1 website?

or is 1 plugin for a set amount i.e. = 2 3 5 6 or 9 websites?

Love to hear from you if you have the answers, so much going on and so busy getting ready it seems a hard one for a quick answer.

thanks

shaun ??