Can I buy a phone directly from the apple store and then get it activated by boost mobile and keep my same phone number?

You should be able to, as long as you buy an unlocked phone directly from Apple. That is the only place you can get a truly unlocked phone that will work with any carrier, and will not lock to one upon connecting to their service.

I did this somewhat recently with an iPhone SE 3rd gen to an iPhone 14 Pro. I bought the phone from Apple, then called the carrier (Consumer Cellular at the time) to have them transfer my phone number from the SE to the 14 Pro. Once completed, the SIM in the SE was no longer active, and the eSIM in the 14 Pro took over.

Yes as long as you know your boost info then they will activate your phone with the same number

Then I would borrow someone else's phone so you can call Boost to confirm.

Although, since you're online now, the answer is right on Boost's site:

https://help.boostmobile.com/docs/number-transfer-guide

You are asking neither Apple nor Boost Mobile here, but other end users. My best advice to to start with asking Boost.