The data would also have to be stored not on SoSciSurvey but on an external secure Server.
That would mean that you installed SoSci Survey on a local server. Given that our limit for free academic surveys is 5.000 respondents, and the costs for paid projects on our cloud service increase with the number of respondents, that's reasonable.
On s2survey.net (our professional server) we had a survey last year with ~250.000 respondents. However, they were spread throughout the EU, so different time zones flattened the rush hours.
Actually, SoSci Survey is a very efficient software - but honestly, that's to a large part the merit of MySQL, the database underlying SoSci Survey. To poll 50.000 respondents per day (including the spikes over time), you should run a VM with 4 vCPUs, 4-6 GB RAM, on a host system with fast data storage (server SSD). These is a rought orientations, of course. It depends on the setup and the overall database size, and on the questionnaire whether 50k is is at the upper limit or of 100k or 200k will run fine, as well.