If you bought a ticket for (say) $400 for 2 people and cancel the trip, you’ll get a voucher for $400. You can use it however you want, either for 1, 2, 3, or more.
Most e-vouchers are good to be used many times until used up, but some are one-time use only, (Using the above example, if it was one-time use and you bought a $25 ticket, you would lose the remaining $375 value. If it was the other kind, you would get a new voucher for $375.)
So note the rules of the voucher you receive.