You can draw one ticket for each purchase that you make.
You have to buy something else, then get in line again if you want to draw a second time.
There are only so many tickets available.
この場合、次のような言い方ができますよ。
ーYou can draw one ticket for each purchase that you make.
「１会計につき１回くじを引けます」
こう言うことで一人１会計１回というニュアンスになりますよ。
ーYou have to buy something else, then get in line again if you want to draw a second time.
「２回目を引きたいなら、何か他に購入して並び直してください」
to get in line で「列に並ぶ」
ーThere are only so many tickets available.
「くじには限りがあります」
ご参考まで！
カナダ 
