The original object can be iterated over and the length of each property value can be determined...

const comments ={
"a":[
    {
        "a":"b"
    }],
"b":[
    {
        "a":"b"
    },
    {
        "a":"b"
    },
    {
        "a":"b"
    },
    {
        "a":"b"
    }]    };

let count=1;
for(let key in comments)
{
    console.log(`${count}. Length: ${comments[key].length}`);
    count++;
}

You can use this approach without setting counter. I think the code is explaining itself, but if you need any further explanation i'm here to help.

const keys = Object.keys(comments);

keys.forEach((key, index) => {
    console.log(`${index + 1}. Length: ${comments[key].length}`);
});