How does the student recruitment process work? Do students apply through a common pool or do they apply to specific labs? Do they rotate in different labs before settling on the lab of choice?
Each Institution has its own selection procedures- usually annually, and in some cases twice a year. Some places offer completely free rotations, others restrict them based on density of students in labs, or similar criteria. Other Institutions have students matched up to labs as part of the interview process, with PIs writing up projects and students selecting from them and handing in an order of preference. These procedures can be changed by the members of each Institution, and indeed have been changing over the years. So contribute to the process once you join—the same strategy may not work as well from one institution to another.
Some Institutions have restrictions on number of enrolled PhD students. However there are few restrictions on number of JRFs (Junior Research Fellows, grant funded research support staff, who can do work ranging from technician type of work to handling a research project by themselves—its up to the PI of the grant). JRFs come for anywhere between 1-3 years (2 being average) and can also pay themselves out of their own independent fellowships if they have qualified for one such as the CSIR/DBT/ICMR/UGC fellowships. These students are highly motivated because they want to get into a PhD program, often outside the country.