The sentence is grammatically correct, but not natural. It's unnatural because it's not clear what you mean. As Austin says, it could mean several things. I may be wrong, but I think you meant, "We have so much things to talk about when we are alone together".

The sentence is a little ambiguous. Do you mean: "We have so many things to talk about regarding being alone together" OR "We have so many things to talk about while we're alone together" The first one means you are talking ABOUT being alone together. The second one means you will talk about something WHILE you are alone together.

Hi Ethan, you can say "We have so much to talk about" or "We have so many things to talk about.

Thank you for your correction, Annie. Your comment makes me realize that my original sentence does cause some misunderstanding.I will keep that in mind.

"We have so MANY THINGS to talk about" OR "We have so MUCH to talk about." "Many" is used with countable nouns (like "many things"). "Much" is used for non-countable nouns. For the rest of the sentence, maybe you mean: "We have so many things to talk about when we are alone together" (This person is easy to talk to and you always have things to talk about when you are together) OR "We have so many things to talk about because we are alone together" (You a