Statistically how many unconfirmed transactions get rejected?
For a transaction to be included in a block, the probability is related to the fee that transaction pays to the miner (higher fee = more likely) and varies over time based on demand of block space (more congested = higher fee needed). It's also unpredictable because miners can choose whatever transactions they want to, doesn't always have to be the x highest fees. 

Here's some stats on current fee vs confirmation time estimations: https://bitcoinfees.21.co