Dear Sir,
As per your question, i undestand that, your are manufacturer in mumbai and supplies goods from mumbai to Dehli for trading.
For obtaing the Delhi GST and Mumbai GST - Your turnover must be crosed the prescribed limit in that state.
For obtaing the Delhi IGST and Mumbai IGST - You have to obatin the IGST registraion only in the state from where you make Inter-State taxable Supply (Only outward ). In the given case, the supply of goods from Mumbai to Delhi is considered as Inter-state supply for the purpose of GST.Therefore, you must be requried to obtain IGST registration in Mumbai and in case of Delhi only if you make Inter-State supply out side the state.