Therefore, I was thinking about going to my favourite Korean restaurant next week, but I can’t go now because of the Coronavirus.

"Therefore" sounds a little strange in this context. Leaving it out sounds better.

The amountnumber of people infected with the Ccoronavirus in Spain is increasing again.

We tend to use “number” when it refers to something countable.

Therefore, I was thinking aboutof going to my favourite Korean restaurant next week, but I can’t go now because of the Ccoronavirus.

“Therefore” suggests that you’re introducing an argument or conclusion that follows. While I suppose you are indeed doing that, it doesn’t make sense here because it sounds like “I was thinking of going to my favourite Korean restaurant” is the result of what is described in the previous sentence. If you wanted to keep the “therefore,” you could instead write something like: “Therefore, I had to give up on my idea of going to my favourite Korean restaurant next week.”