人数は減るとは思うけど、そんなに多くないと思うって英語でなんて言うの？

人数 - number of people
減る - go down, decrease
人数が減る - number of people go down, number of people decrease

〜〜と思う - I think

そんなに多くない - not that big, not that much

I think the number of people will decrease, but I dont think it’ll be that much
I think the amount of people will go down, but not by much