Contribution indexing was introduced in 2018. From December 7, 2010 to January 29, 2018 the maximum contribution was $2,500. Prior to that it was $600.
The maximum contribution is indexed to the Year's Maximum Pensionable Earnings (YMPE) and will change January 1 each year. SPP will release the updated contribution amount on December 17th of each year.
Member's making contributions to their SPP account must have available RRSP room. This amount can be found on your Notice of Assessment provided by CRA.