Does the AMT Tax Compound?

I have ISO shares. If I exercise my options I understand I must pay an AMT tax:

https://blog.wealthfront.com/when-to-exercise-stock-options/

My question: I plan on exercising my options and I will pay the AMT. But is the AMT triggered multiple times if my company raises subsequent rounds, or am I only subject to the tax once at the time of exercise?