Hello,
Yes only 1 loopback interface called lo0 is available in JUNOS.
However, one can create many units (how many is platform dependent) on lo0 and put these units into different routing-instances.
This means that if you don't have routing-instances at all you can use only single unit on lo0. You can also assign many IP addresses to this single unit if you need these IP addresses to be always reachable.
BTW, loopback interface in JUNOS does not really loop back packets, unlike IOS loopback interface. Implications are: you cannot have NAT assigned to JUNOS loopback, you cannot have a dial-peer pointing at JUNOS loopback etc.
In a nutshell, JUNOS loopback and IOS loopback interfaces are fundamentally different.
HTH
Regards
Alex