There is no limit to how much you can contribute to a 529 plan.* As others have said, there is a limit to how much you can deduct on your state return. See:
https://leg.colorado.gov/sites/default/files/te6_colorado_tuition_program_529_deduction.pdf
*See this discussion about 529 contributions and the requirement for a gift tax return:
https://www.savingforcollege.com/article/dont-worry-too-much-about-the-annual-gift-tax-limit
