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lokesh lokkilokesh lokki 

Only one Project can be active at a time, if we are creating other Projects against the account those should be marked as Inactive.

VinayVinay (Salesforce Developers) 
Did you try using workflow rule or process builder?  Create a rollup summary field if its a child object on parent to count number of project existing and if rolup field is more than 1 you can trigger WF or Process builder to update field as Inactive.

Thanks,