“There's a good chance that you'll end up buying a lot of stuff while you're there.”

Your roommate is going on a trip to a foreign country that she's never been to before. You've been there, so you're giving her advice on what to pack. You think she should pack light, so you give her a reason not to take a lot of stuff with her:

There's a good chance that you'll end up buying a lot of stuff while you're there.

there's a good chance that (clause)

When you want to say that something might happen, and the possibility is pretty high, you say "there's a good chance that ___" For example:

There's a good chance that I'll get lost on the way, but I'll call if that happens.

There's a good chance that our refrigerator is going to die soon, so we might as well get a new one.

end up (doing something)

The phrase "end up ___ing" is used to tell the final result of something.

In the example above, the speaker says that her roommate will "end up" buying a lot of stuff. This means that, even if she isn't planning to buy a lot, she will buy things anyway once she gets to the foreign country.

a lot

English learners often use the word "many" to describe a large number of something. But "many" is quite formal and more often used in writing than in speaking. (Although "not many" is very common in both.)

In spoken English, "a lot" is the normal phrase for expressing a large number or amount:

There were a lot of people there.

We've got a lot of homework tonight.

stuff

"Stuff" can mean almost any physical items or material. It's similar to the word "things", but "things" are countable and "stuff" is not. So "stuff" is better to use when you're talking about a group of things that are different from each other - different sizes, shapes, etc.

"Stuff" is especially used to refer to things that you buy or own:

We went to the mall and bought so much stuff.

Did you get any stuff for dinner tomorrow night?

I have way too much stuff in my apartment.