How to express a plural of the cardinalities of sets?

Question

I express variables $x_1, x_2, \ldots$ as "$x_i$'s".

Similarly, when I want to express sets $\mathcal{S}_1, \mathcal{S},2, \ldots$, I also use $\mathcal{S}_i$'s.

However, when I want to express the cardinalities of the sets, how to use??

1. $\lvert\mathcal{S}_i\rvert$s ?
2. $\lvert\mathcal{S}_i\rvert$'s ?

I thought at first to post this question on English question board, but because I want to hear the answer from mathematicians, I post here. If the topic is not right, let me know. I will delete it immediately.