Learn Italian: Questi turisti non capiscono la rigidità del confine. - These tourists do not understand the strictness of the border.

To emphasize the strictness, you could exclaim: Che rigidità del confine che non capiscono questi turisti! (What strictness of the border these tourists don't understand!). In spoken Italian, adding that second, grammatically unnecessary che (the "pleonastic" one) connects the exclamation to the rest of the sentence while adding strong emotional emphasis.

The singular is turista (ending in -a). Why does the plural end in -i here?

Nouns ending in -ista (like turista, dentista, giornalista) are the same for both masculine and feminine in the singular. However, they branch out in the plural: they take -isti for a masculine or mixed group, and -iste for a purely feminine group. The masculine plural adjective questi tells us this is a masculine or mixed group of tourists.

If we were talking about multiple strict rules, how would rigidità change in the plural?

It wouldn't change at all! Italian nouns that end in an accented vowel (like rigidità, città, università) are invariable. You only change the surrounding articles and adjectives to indicate the plural, for example: le rigidità.

Could I use frontiera instead of confine here?