a better term to describe that number is "depth" instead of number of confirmation. it says it all, when you see a tx that has 30k confirmation, it means it is 30k blocks deep from the head. so if for example the head is at block height 10 and your tx is in that block, your confirmation is 1 meaning it is 1 block deep. when there are more blocks found and we reach block 15, your tx has 5 confirmation and it means it is 5 blocks deep.

what is the most number of confirmation? it is equal to number of blocks we have at that time (plus 1*).
last block's height: 597211†
tx with highest number of confirmation (597212†): https://blockchair.com/bitcoin/transaction/4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b

* the reason for +1 is because the very first transaction is the genesis block's coinbase tx which is in block #0
† these numbers could be different by the time you look at them since the blockchain is constantly growing.