If a customer installs and purchases, how many postbacks will the advertiser get from Apple?

Apple will only ever send one SKAdNetwork postback per install.

If an install is the furthest a customer goes through the conversion funnel within Apple's conversion timer windows, then you as the advertiser will received an install postback. If a customer also purchases within Apple's conversion timer windows and you decide to register this as a conversion event with a conversion value, the postback you receive will have the appropriate conversion value attached, and this can also be counted as an install.

Once a postback has been sent by Apple, no additional postbacks will be sent for post install conversions. Therefore, if a conversion does not happen within Apple's windows then you may never see a postback with that conversion information to be attributed to the campaign.