Hi @emanabdelhalim 

Refer to the Help Manual

https://help.nintex.com/en-US/nintex2019/current/sp2019/DatabaseDesign/MonitorWorkflowData.htm

You need to purge the Workflow data. Refer here

https://help.nintex.com/en-US/nintex2019/current/sp2019/DatabaseDesign/PurgeWorkflowData.htm

Pls read the entire document first. Avoid reading one part or section and immediate take action. 
If you are new or this is your first time, Perform a backup first.

thanks @Garrett  for your replay, i already read this articles before but as i mentioned purge data not reduce rows on WorkflowInstanceTasks table it still the same

i need steps to clean this table WorkflowInstanceTasks something like the Purge or shall i delete them directly from database?

Same issue here