can I claim maintenance all once or have to take it on monthly basis?

Firstly, as you have stated that you have caught him having affairs, you can register a complaint against your husband for adultery under Section 497 of the Indian Penal Code.

Secondly with respect to the divorce, if he is not agreeing to your terms, then you can claim divorce under Section 13(1)(i) of the Hindu Marriage Act which states that Any marriage solemnized, whether before or after the commencement of this Act, may, on a petition presented by either the husband or the wife, be dissolved by a decree of divorce on the ground that the other party has, after the solemnization of the marriage, had voluntary, sexual intercourse with any person other than his or her spouse.

So you need not apply for divorce by mutual consent if you wish to. 

Along with this petition you are eligible to claim maintenance for yourself and your sons. This can be done under both Hindu laws as well as under Section 125 of the Indian Penal Code. 

On separation, the wife has right to 'Stridhan' and all jointly-held investments where she has made some monetary contributions. Stridhan refers to the woman's personal wealth and encompasses all movable and immovable properties, gifts, and everything else received by her from her parents, relatives and even from her in-laws at the time of her marriage. It can include things like cash, gift-items, jewellery, car, furniture and more. She also has a right on all gifts received after her marriage, for instance those given to her on festivals and other functions.

It is beneficial if you have records of the same as, if the ex-husband refuses to part with items that come under 'Stridhan', you can back them up with relevant bills and receipts whenever needed. For this, it's important that you make a list of all your assets and gets it signed by two witnesses so that there is no room for it to be challenged later.

If the husband still refuses, and it is proven beyond doubt that he is holding assets that legally belong to the wife, he is liable for criminal prosecution.

So therefore, when you file for divorce, you can specifically claim a share in the property plus either monthly or one time alimony / maintenance from your husband.