Final answer:
The corrected words in the sentence are 'number' to correctly refer to people, and 'stationery' to refer to writing materials instead of 'stationary'.
Step-by-step explanation:
The two words that are used incorrectly are "amount" and "stationary." The correct word for referring to the number of people is "number," not "amount." Additionally, the word "stationary" refers to not moving, while the correct term for writing materials is "stationery." Therefore, the corrected sentence would be: After determining the number of people she wanted to invite to her party, Krysta went to the stationery store to buy invitations.