Why do i require self ip and floating ip ? if i am configuring HA and have 5 VLANS in active device , do i need to have 5 floating IPs ?

asking the question once is enough i believe :)

you don't need it, without it nothing will break directly. but things will become complex. if you for example have a server with the big-ip as default gateway, what IP do you configure? you have to choose one, so either unit A or unit B. if you have a floating Self IP that IP moves to the active unit, so you can use that one.

similar with traffic from the BIG-IP to your server. without a floating self IP it will use the non floating self IP, which will change with a failover, perhaps causing issues.

so no you don't NEED it, but it makes things much nicer.