Q.How do we come to know the maximum safe anchoring depth for the vessel?

It will depend upon windlass capacity. The power it has.

The manual/drawing will given wait of chain plus anchor.

With that we need to calculate how much hanging wait (of anchor plus cable )windlass can lift. Also make allowance to be made for the break force required to take anchor out of embedment in sea bottom.

Usually thet yard engineer can give all that caluclations.

For handy max bulker its usually 4 shackles hanging windlass can lift plus after allowance for pull force needed to break anchor from sea bottom its usually 80 mtrs maximum.

But we should get ship specific information and company policy also.

Some companies SMS gives clear instruction for their ship types.