Hi Parag! After offboarding around 8 pm, you would be at a near-perfect timing to catch a Volvo bus from ISBT/Mandi House/Majnu ka Tila towards Manali. On an average, buses to Manali drive every few hours from Delhi till 10-11 pm at night.
You can book a ticket in an volvo from ISBT, or can go via state transport buses too (a cheaper, less comfortable yet worthy option). Also can consider booking a cab using any app.
There are so many Volvo Buses Running from Delhi to Manali in Evening. So you can take bus from Majnu ka Tila, New Delhi and Reach there. Hope You will get the proper answer regarding your question. Happy Journey..!