There is an affinity between Imperfective aspect and plural objects.
1. Ћн чђсто писђл (Impf.) мнљ пќсьма. He often used to write me letters.
2. Ћн написђл (Pf.) мнљ письмћ. He wrote me a letter. (He got the letter written.)
3. Ћн написђл (Pf.) пќсьма. He wrote the letters.
In example (1), above, there is no definite end-point; he wrote a number of letters a number of times.
In example (2) there is an end-point: one finished letter.
This affinity is somewhat vague; it is a statistical fact rather than a grammatical rule. It is quite normal to have plural objects with Perfective verbs as well, as in example (3).
But notice that the English translation of (3) contains the article the, which renders the meaning of definiteness in much the same way that the Russian Perfective написђл renders the meaning 'end-point' (a kind of definiteness). The translation 'He wrote letters' for (3) would be wrong; this translation matches the Imperfective verb in (1), not the Perfective verb in (3).
The reason that Perfectives go more often with singular objects than with plural objects has less to do with grammar than with real life; when someone wants to sit down and write a letter, or eat a bowl of kasha, or finish an assignment for class, or get something done, it is more likely that it will be one thing rather than many things, though there is no rule against writing more than one letter at a time, eating any number of bowls of kasha, or doing six assignments at a sitting.
The other element of plurality in (1) is the adverb чђсто 'often, frequently, many times'. The meaning of this adverb contains the element of plurality. The same can be said for the adverb всегдђ 'always'. The following is a rule of thumb:
Use the Imperfective aspect for repeated or habitual actions.
In the following examples the Imperfective verbs are in italics:
Когдђ ї занимђлся рџсским языкћм, мІ чђсто читђли по-рџсски, писђли упражнљния и переводќли тљксты с рџсского языкђ на англќйский. When I studied Russian we often read in Russian, wrote exercises, and translated texts from Russian into English.
Ї чђсто хожџ к врачџ. I often go to the doctor's.