What is the maximum number of water molecules that has been recorded till date to present as water of crystallization in a compound?
It might be a protein crystal. Proteins are large and the interstitial space is filled to a large extent with water. The volume of a protein crystal occupied by the solvent can be 50% or so. However the fraction of waters observed in crystals (relatively fixed) is often low, on the order of 10%.
For instance, from a report on the crystal structure of crambin (chosen arbitrarily, see Ref. 1):

The volume of solvent (32%) is low but not atypical for proteins [values of 30-78% solvent are found.] Crambin is small (Mr 4720), and most residues are solvent accessible. Although crystals were grown from 60% ethanol, only 7%(vol/vol) ethanol has been located. Of the 85 water molecules expected (based on the density and the presence of 2 ethanol molecules), the model includes 64 [...]

It's not clear whether the 85 expected or the 64 found water molecules would be considered crambin's water of crystallization (presumably all count, even if not observed).
As another example, the model crystal structure of BPTI contains 63 waters (observable waters, see PDB structure 5PTI).
While I was looking for the protein crystal structure with greatest number of waters in its model, Karsten Theis suggested as an example the pyruvate-bound crystal structures of ADP–glucose pyrophosphorylase from the bacterium Agrobacterium tumefaciens (pdb 5W5T, see the file for a description of the solvent model), which has 3752 waters in the crystallographic model (59.12% of the volume).
Many waters may have irregular positions, and some may form clathrate-like solvation structures (as in gas hydrates, as andselisk comments), and some can be placed at regular locations.
References

M. M. Teeter. Water structure of a hydrophobic protein at atomic resolution: Pentagon rings of water molecules in crystals of crambin. Proc Natl Acad Sci U S A. 1984 Oct; 81(19): 6014–6018. doi: 10.1073/pnas.81.19.6014