This part of your post is not clear:
I went to relationships and established connection between CalcTBL::TotalNumberofOrders and MySQLTableName::extra_stock and MySQLTableName::to_order.
Relationships should match pairs of fields of the same type of data. You appear to have three fields in this relationship. Exactly how did you set up this relationship? What field matches to what field? Do they match with the = operator? (A fileMaker relationship is set up much like the "join" part of a SQL expression. You can think of them as expressions starting with Select *, defining a "join", but not having a "where" or "orderby" clause.)
In field type I chose "Calculation" and in option using Sum function I added 2 fields (extra_stock and to_order) from MySQL together.
Can you post the exact calculation used? It looks like you may be confusing the fields to be used to match records with the fields to be used to compute a value in your calculation.
...Inventory Tab in filemaker is a tab showing product details
And that tab control is located on a layout. On what table is the layout based?