Hi,
how many instances of mst can be done on a swicth
Depend on the switch model you are using.
If the number of VLAN in the network are more you could run less STP instance by grouping the VLAN.
And for ALL the VLAN in the mst instance there will be 1 Root you cannot have individual root for the Valn in 1 instace.
Router(config)# spanning-tree mst X root [primary | secondary]
Chao
Vishwa