I've been texting with this Bumble girl, the conversation was quite sexsual from the begining and after like only 3 texts that we exchanged she writes me: "i really want to have sex with you".

I will just add that the conversation streached over 3 days, i usually send one message a day on dating apps ( i'm really busy).

Not sure if she's serious or just teasing me.

What should be my next message?
I was thinking to just send her the following message: "what is your number?" and to keep it from there on the phone. Is that the right thing to do?