You can definitely purchase a subscription for 1-2 months.
In order to do so, you can purchase a subscription and select Billed monthly at checkout. When you do not need to use BioRender, you can go ahead and cancel before the next billing date.
Below is a gif of what you would select for monthly billing for an Academic Individual subscription:
Please contact our support team via the chat box in the bottom right corner if you have further questions.