This does depend on the insurer, and the type of policy, you choose. Insurers will generally cover between 20 and 60 critical illnesses in their policies, and let you claim once, after which the policy will end.
The payout amount can also vary as some insurers may make a smaller payment, for example, whichever is the lower of 25% of your total cover or £25,000.
This usually applies if you get diagnosed with specified less severe illnesses. In these cases, you’d expect to get the payout and the policy to remain active.