How do i add more monsters to my line up, i got the ocarina that allows 4 monsters in my line up, but when i go into battle i only start with 3?? I have two other monsters but they only come in when one of my first monster dies.

That’s how the game works. Ocarinas and musical scores add to your line-up, only. That is, they let you put in monsters for backup that come in after yours die. 

You can’t have more than 3 in a fight at a time if you don’t count First Ones. 

Ooo ok, so basically they just give u more back up, but you will always start with only three. Thanks for ur help