Yes, you can have just one cable between two switches or as many as you have unused ports available. I assume you have SM (subnet manager) enabled on at least one of the switches and/or on at least one of the hosts: SM will figure out how to use links between switches and default IB routing algorithm should be just fine for this simple topology.
The number of links between switches will define “blocking ratio” of this config. For example, if you will use 18 links between switches and will have 18 servers on each switch connected – you will have “nonblocking” (i.e. 1:1) config since all 18 servers on the first switch could connect to all 18 servers on the second switch with full BW. If with same number of servers you will use only 3 cables you will get 1:6 blocking.