Do I understand correctly that you want to service around 250 tenants using the single VNF instance? I'd say the best practice is one VNF instance per tenant, and scale-out scenarios where multiple instances are used for tenant are supported; it looks like you want the opposite - one VNF for many tenants?
If it is so then it will potentially work, although about the scale you have to either discuss with Juniper SEs or maybe just test it.
I see 2 possibilities:
1) VNF in L2 transparent mode, having 2 interfaces (left and right). Such VNF can be used in multiple service chains for multiple tenants, the traffic will be separated using VLAN tagging.
One possible problem with such scenario that I see - VLAN tag is set by Contrail and VNF will not easily know which VLAN corresponds to which tenant. So it will work if all tenants use exactly the same service, not sure what if otherwise.
2) VNF in L3 ("In-Network") mode. You can create multiple pairs of interfaces on VNF, and configure multiple service chains (for different tenants) in Contrail to use different interface pairs. This is possible with what they call "service chaining v2".