Well as an employer it is your responsibility to ensure you have hired someone suitable for the job in the probationary period. Any further dismissals should be subject to a disciplinary policy and breaches of specific targets on multiple occasions as agreed with the employee.

Having a hard limit of 5-x% of staff to remove on an annual basis is indicative of either megalomania or a complete failure of hiring policies.

It is certainly within the reasonable to assume that on certain occasions all staff will be performing admirably and will not be required to leave; however a hard cut policy would require sacking perfectly fine employees.

This is the kind of shit that happened in old wall st brokerages.

Further to this European countries employment legislation would render this most likely illegal. I've worked at European start ups that became mid size companies and there were regular culling based almost entirely on the whim of the CEO who didn't like the cut of their jib. Many of these resulted in several very expensive court actions for the companies involved.

Furthermore the culture such a policy would foster in the workplace would most likely be completely destructive and demotivating.