not show all record from firestore

I have fire-store collection with the name of "conversations" in my DB i have 300+ record but when i use below query i get sometime 90 record some time 105 record. i check every thing for any limitation but there is not set any limitation in my account. so, why its not show all record

const conversations_query = db.collection('conversations').orderBy('lastMessage.timestamp', 'desc');
const snapshot = await conversations_query.get();
let firestore_data = [];
let count = 0;
        snapshot.forEach(doc => {
            firestore_data.push(doc.data());
            count++;
        });
        
        console.log('Total_number_of_records=>', count);