See https://www.drupal.org/docs/core-modules-and-themes/core-modules/jsonapi-module/pagination#s--cant-i-set-a-page-limit-higher-than-50:
Why can't I set a page limit higher than 50?
First, read the example given above. Understand that JSON:API must run
individual access checks for every entity in a response. Second,
understand that the JSON:API module aims to be "zero configuration."
You shouldn't have to install, alter or configure anything to use the
module.
The reason for this is to protect your application from a DDoS attack.
If a malicious API client set a page limit of 200,000 resources, the
JSON:API module would need to run entity access checks for every one
of those entities. This would quickly lead to out-of-memory errors and
slow responses. The server needs to set a maximum. The limit of 50 was
somewhat arbitrarily chosen as a nice round number.
Please understand that there have been many long conversations around
this decision and a compromise had to be made between support burden,
sane defaults, and frontend performance. While the JSON:API module
maintainers do understand that this may not be ideal for every use
case, they're confident that if your client follows the
recommendations in these docs, it should have little to no impact on
you :)
There is the JSON:API Page Limit module and also JSON API Defaults (part of the popular JSON:API Extras module) to change the limit.
After you have installed the JSON API Defaults module go to
http://example.com/admin/config/services/jsonapi/add/resource_types/taxonomy_term/[vocabulary]
and change the page limit at the bottom of the config form.
Then you can request a higher number of items with the query parameter page[limit]:
http://example.com/jsonapi/taxonomy_term/[vocabulary]?page[limit]=1000
