19 of them (has to be an odd number). When asked this question, Chris said that there would be a selection process for the 50k nodes and that older, known members of the community would be chosen first, with any free spaces going to other applicants on a first come, first serve basis. These particular Masternodes need to be online 24/7 and with a lowish number the Authority aspect of Proof of Authority (where you are putting your identity on the line as well as the collateral) matters more than with a large number of Masternodes. Moreover, it seems to me that the 50k masternodes will receive the same income as the 10k masternodes (don't hold me to this, I don't know this for sure), so 50k masternode operators will need a reason above and beyond the income for running one. Thus they will be choosing people who they can be quite certain will be able to maintain a node 24/7 and who will be happy to front the 50k collateral when they could receive a better ROI from a 10k node. This is more likely to be true of larger holders than smaller holders, and larger holders are in turn more likely to be earlier investors.
The plan is to introduce dynamic membership for these 50k masternodes as well. This would change the selection process completely, and allow anyone to run a 50k masternode, so long as there was a spot available. There are plans to increase the number of available spots, so it may well be possible for anyone to operate a 50k node whenever they want down the line.
Thanks for the detailed explanation, I hope they take into account staking record too, not just known members of the community.