I use Jira Cloud API https://developer.atlassian.com/cloud/jira/platform/rest/v3/?utm_source=%2Fcloud%2Fjira%2Fplatform%2Frest%2F&utm_medium=302#api-rest-api-3-search-get.
But I have 100 tasks per load limit. My project has 5000 issues. It’s 50 requests and it is very long for me.
How can i load all project issues at once?
You can’t. Paginated REST APIs are a common design pattern for a good reason.
Imagine Jira would have to load all your 5.000 issues into memory, transform them into JSON and send you the response (possibly being a few MB in size).
How does jira download and display issues in interface? I want to download 500 issues, is there any way?
The API limits are the limit. There should be no way to bypass them.
You can try using the Jira expressions REST API, which has a limit of 1k issues loaded with JQL for further processing.