I didn't expect to...,

[Silverobama]

I was planning to host an English club last night. The shop owner told me there might not be many people, since it was the first time. But when I got there, I saw a lot of people, and I said to them…

I didn't expect to see so many people tonight.

Is what I natural?

 

[5jj]

Is what I said natural?

It is, though you missed a word in your question.

 

[emsr2d2]

I was planning to host an English club last night at a local shop. The shop owner told me there might not be many people no comma here since it was the first time meeting, but when I got there, I saw there were a lot of people. and I said to them…

Note my changes above. It would make sense to mention the location in the opening sentence. In your original, my first thought when I saw "The shop owner" was "What shop? I thought we were talking about an English language club".
Although I've left the repetitive nature of "there might not be many people" closely followed by "there were a lot of people", it's awkward. In real life, I'd make that middle sentence:
The owner said there might not be many attendees as it was the first meeting but he was wrong - lots of people came!

I didn't expect to see so many people tonight.

I might say "I didn't expect to see so many of you!" to personalise it to the attendees a bit.

Is what I said natural?

There's your missing word!