Apologies newbie question

I have two companion vouchers and 420k Avios.

I want to book for a family of 5 to fly to Orlando next August.

Searching for availability seems to be a dark art ! My question is do I need to find Avios availability for 5 seats or just 3 (then use the two compion vouchers) ie to use a companion voucher does it use Avios seat allocation or general seat allocation

hope that makes sense

any my other advice or tips would be greatly recived

thanks

michael