Can I buy multiple plans at a time?

Yes, you can.

When you buy multiple plans at a time, the plans get used one after the other.

To elaborate, say you bought 2 plans on Jan 1st. Then the plan 2 never gets used until plan 1 is completely used up or expired due to validity. The validity of plan 2 starts only after it’s first 2000 page views are put to use.