Unable to agree on a plan to replace the ACA, Trump and others propose to repeal elements and decide later what to do.
The Congressional Budget Office has estimated impacts of such a plan. Repeal and delay leaves the fate of the individual market completely in the dark, because it’s unknown what kind of subsidies would be available for low and moderate income people to buy insurance in the yet-to-be written replacement. The CBO estimate assumes nothing is done to replace it, which would mean premiums would increase by 50% more than would be expected under current law by 2020, and they would about double by 2026.
Read about how the CBO estimates the action would cause 32 million to lose their health coverage by 2026, and 17 million to become uninsured next year in the summary.
Read the cost estimate by the Congressional Budget Office