Can we only buy 1 month not package for 12 months?

Yes, when you upgrade, you are actually given options for monthly, yearly and bi-yearly.

How-to-Upgrade-My-Account

Please note though that computed monthly prices will differ with the longer terms.

Also, please note that subscriptions are automatically renewed. If you only want one month, please make sure to downgrade before your next renewal date.

How-to-Cancel-a-Subscription-and-Downgrade-Back-to-Free