In the US, 3 lots (as a minimum) is taught as the general 'rule of thumb'. Certainly each industry, company and product / process will have its own unique characteristics to be considered. And certainly your regulatory agent may have different and more specific advice or requirements...
but that brings up the very good point: for the OP to get a usable answer more specifics are required about his exact situation. To simply ask 'what is the sample size for PQ' is a very vague question...and since Process Validation is not a fully developed and firm regulation with crustal clear requirements for the FDA, let alone other industries that utilize the IQ, OQ, PQ concept for process validation, there are few firm answers and even fewer correct answers that can be given.