Is there a limit to the number of jobs or customers I can have?

With a paid account there is no limit to the number of jobs you can create. For the free Solo plan, you can create 25 jobs per month.

You can create an unlimited number of customers, no matter which plan you have (this includes the free one).

Once created, your job and customer recrods will always be accessible, even if you downgrade to the free plan (with the exception of data related to features which are specific to a paid plan e.g. forms).