How many formula field we can create per object in salesforce.?
There is no specific limit on the number of formula fields you can have on an object, so the limit is effectively the same as the number of custom fields you can have on an object.
That limit depends on your edition and is documented here
PERSONAL: 5
CONTACT MANAGER: 25
GROUP: 100
ESSENTIALS: 100
PROFESSIONAL: 100
ENTERPRISE: 500
UNLIMITED AND PERFORMANCE: 500
DEVELOPER : 500
Installed packages from the AppExchange can push you up to a max of 900 fields total.
The practical limit to formulas though comes from the number of object references you can have per object. That limit is 15 (can be increased to 20 if you open a case with Salesforce), and those are shared across formula fields, validation rules, and perhaps a few other things.
Account.Id and Account.Parent.Id would, taken together, count as 3 object references the last time I had to worry about reducing that number on a particular object. Referencing fields without traversing relationships does not use an "object reference".
