We have only seen six (five fractions and the main monolith), which makes sense as we've only seen 6 heads of Hydra at any one time - in the episode Aftershocks, there are Whitehall, Dr. List, the Sheikh, the Baroness, the Banker and Octavian Bloom. Gideon even tells Ward in Many Heads, One Tale that there are five... However, if you look at the pictures of the monolith, there are more than five holes - here's a picture with 11 showing, not to mention potential others on the back sides, or the top or bottom:
However, we see in both Many Heads, One Tale and Closure that each of the monolith fragments also create portals, as they also liquefy at random times without the key vibration. The only reason we see that five of them are used in Closure is to create a portal big enough for a man to jump through. There happen to be little lines on the platform where the shards are placed showing each shard's "splash radius", so it is clear that Hydra over the centuries has figured out as much as they can about the monolith, and know that five shards are all that are needed for a man-sized hole.
As for why they need the shards, note that SHIELD had to literally cut a hole into that castle to get the monolith out the first time (which Coulson later decided to skydive through)... They undoubtedly figured out the shards are infinitely easier to travel with, and also to thus keep safe, spread out amoungst the heads of the Hydra and their respective vaults (as we see in Many Heads, One Tale).