shinkansen reservations

Hello there,
i have some doubts about making reservations for the shinkansen when i buy the JR pass.

in april i will be traveling to japan with a group. we are 11 people in total. we will buy the jr pass for 14 days.

when i buy it online and receive the voucher, can i already make the reservations for the trains? is there a way to make the reservation before activating the pass?

we are concerned about the availability of the shinkansen in april, and we would like to have seat reservations in advance. is that possible?

thank you so much for the help!!