We can't offer you an exact work item limit as this depends on how your SLA is configured, the number of SLAs utilized, the configurations, how many goals are in each SLA, and many other parameters.
We strongly recommend you generate your reports with low work item numbers in the UI and create background reports for those with high numbers.
As a rule of thumb, always use as many filters as you can! Creating as many different reports as possible is also a good option.
To ensure that the report generation process runs smoothly, always try to fill out either Project, JQL, or Filter Type, since these will have a significant impact on the generation performance of your report.