The way the survey link will be distributed is that many people will see and will be able to click on it at the same time.
The question is: will they actually click at the same time or not?
If you want to be absolutely sure, you will have to use a random generator that takes care of an equal distribution at the time when the respondents start the questionnaire and you have to adjust the random generator very early (and a few times) to respond to drop-outs.
It is helpful if the randomization does not occur at the very beginning, but after one or two pages. Because the very most respondents drop-out on the first two pages.
If it's likely that responses will spread over time, a random generator that counts only completed interviews will be fine.
And finally, is there a way to set the exact number of respondents for each questionnaire or is this not necessary because the algorithm tries for equal distribution?
The algorithm (it's actually just an urn that is empied one piece aften another, and then refilled) will try to achieve an equal distribution. Therefore, there's no need to set a specific number.