Per the B51 standard, any number of copies of a piping system can be built from one registration (B51-14 4.1.2):

Any number of boilers, pressure vessels, fittings, fired-heater pressure coils, and piping systems may be constructed from a registered design until a change in the applicable Act, codes, or standards invalidates the design, in which case the design shall be obsolete and no further construction to the design shall be made after the effective date of the change as established by the Act.

However, not all provinces have managed to achieve this level of flexibility. The most fundamental stumbling block is that some provinces require the address of installation for each registration. In theory multiple copies of piping systems can be built for the same address, but building it again for a new address is not allowed. In Ontario PSTD registration numbers are issued for piping systems that will be mobile or built repetitively.

While other provinces continue to refine their processes, several workarounds are in use: 1) register your piping system every time it is sold to a new location; 2) some provinces will issue piping systems too big to be a fitting a fitting number allowing it to be built multiple times.

P6947 A BC issued piping registration.
P36879 This Ontario issued piping system does not clearly show the province of registration.
P37373ADD1 An Ontario issued piping system, addended.
PP03373.0 A Piping system registered in Newfoundland
PP0060.7A2 Piping issued in New Brunswick.
PP581-1888 A piping system registered in Saskatchewan.
PSTD38354 A mobile piping system registered in Ontario. The same format is used for multiple build standard piping systems.
“ “ Quebec does not issue piping CRNs. On-site inspection is used instead. Ship the documents that you would use to support a CRN with the piping system for the AI to review on site before granting system start up.
P1405.4 A Manitoba VESSEL – the “P” at the start does not always mean piping.
PP-0701-E-004-P, PP-0701-E-004-P Alberta registered piping system. The same number has been issued to one vendor working on different skids.