How does it make you feel when someone says they are sorry for your loss? We all grieve differently so it’s ok to not know how to reply :) just a thank you is enough of a reply x
It’s okay to not say anything if you don’t have the words right now. People generally understand this is a hard time, and you probably have a lot of folks reaching out. These words, for some people, can trigger more emotions and tears that they might want to avoid. I would not be upset or overthinking anything if I texted someone this and they said nothing back.
I've always just said thank you because it seems like the polite response. After each loss I wished people would just stop, mostly because it felt fake and like something they were obligated to say. People still say it now when they find out the losses I've had and it feels unnecessary, but again it seems like the polite thing to say.
I think it all depends on your personality and who is reaching out. I barely spoke that first week. When this happened to me I received the messages but did not respond right away. It took me about a week or two to respond. I said thank you and maybe shared a bit on how I was feeling, again depending on who it was. Sending you hugs. Focus on your healing and feelings 
Either say thank you or heart react to their message
Just react to the message with a heart
Thank you is enough or not responding until you’re ready. Take your time be gentle with yourself and your heart.
Don’t feel obligated to respond right now. That’s okay.
Just say Thanks or a heart. FIL passed away last year and that’s all we had the energy for. Some people we didn’t even send anything back until weeks later because we couldn’t face to read the essays on how they felt and loved FIL. You don’t have to do anything though x
Thank you, thank you for your condolences, I appreciate you. It depends on the person you’re replying to.