In continuation of the replies from our learned friends, I would like to add that if the concept of probation is properly understood, the necessity for extending a new entrant's probation is a matter of limited choice. As such, the question regarding the probable number of extensions of probation becomes automatically irrelevant.
Understanding Probation
Probation is the situation or the definite period during which a new employee's suitability for their job and the work culture of the organization is assessed objectively. If the incumbent is a fresher and the job for which they have been selected is highly technical or professional, involving intensive training either pre-appointment or on-the-job, probation can be extended only once. This depends on the nature of the shortfall noticed in their performance, subject to the service conditions or Standing Orders of the establishment.
Limitations on Extension
However, in my opinion, the extension cannot be more than the original period of probation, whether it is three months or six months as already fixed. In other words, after the expiry of the probation period, the probationer cannot be kept as such under the pretext of extension after extension, which would amount to bad HR practice or unfair labor practice.