How many people or percentage has held at least 50 or 100 btc at one point?
there is simply no way of knowing this. even in early days if you mined bitcoin you would have done it to a new address each time so there is a good chance the coins are spread among multiple addresses. and at some point people saw the rise, sold or at least moved their coins to different addresses,... i have seen whales do it multiple times. they spread the coins among multiple addresses.
Thus there probably are many would be millionaires even now had they kept the btc?
there are but again not necessarily. many who bought or mined in early days, lost access or sold at some point and some held. so you can't necessarily say if someone had large amounts of bitcoin at some point he is now a millionaire.
