One special use of the oblique plural is to indicate a quantity of some sort.
With Cardinal Numbers
When the suffix –ओं is used with certain cardinal numbers, it means “all X”, for instance दोनों (“both / all two”), तीनों (“all three”), चरों (“all four”), दसों (“all ten”), बीसों (“all twenty”), etc.
तीनों लड़के बहार गए – “All three boys went outside”
दुश्मन ने चरों तरफ़ से हमला किया – “The enemy attacked from all four sides”
The word दोनों is very common, and is used to mean “both” (i.e. “all two”).
मुझे दोनों अभिनेत्रियां पसंद हैं – “I like both of the actresses”
With certain numbers, it indicates an indefinite quantity, such as हजारों (“thousands”), लाखों (“lakhs”, i.e. “hundreds of thousands”).
With Certain Nouns
When used with certain nouns, the oblique plural indicates an indefinite quantity.
काफी दिनों बाद मेरी उससे बात हुई – “I spoke to him after quite a few days”
This construction is often used with other adjectives such as बाद. Note that this is grammatically different than दिनों के बाद.
पिछले कुछ वर्षों में लाखों लोग यहां आये हैं – “Hundreds of thousands of people have come in the last few years”
The Suffix –इयों
The suffix –इयों indicates an indefinite multiple of some quantity.
बीसियों छात्र उसका भाषण सुनने के लिए आये – “Scores of students came to hear his speech”