Yes. It's a good, natural way to say it. Everyone would understand it to mean "We bought approximately ten apples." IN THEORY it is very slightly ambiguous. Without other context, it means "We bought approximately ten apples, but I didn't really count them." With context, it COULD mean "I'm not sure we bought any apples." For example: "A group of us did the shopping. The list said we should buy ten apples. I didn't do the produce shopping myself, that was someone else's job. But since it was on the list, I guess we bought ten apples." Other ways to say it--NOT better, just different: "We bought about ten apples." "We bought ten apples, more or less." "We bought something like ten apples." "We bought around ten apples." "We did buy apples--about ten, I guess."

You could say "I'm pretty sure we bought ten apples".

Yes. It's correct. There are many ways to say this idea.