Looked for this option a couple of times and found some threads discussing it, for example:
Th thread titled “My decks say I’ve completed the deck today but I want to use it again”
I solve the problem according to the manual, like this:
Study by card state or tag
Select a certain number of cards from the current deck to study. You can choose to select new cards only, due cards only, or all cards; after you click "Choose Tags", you can also limit the selected cards by tags. If you wish to see all the cards in the deck (for instance, to study before a big test), you can set the number of cards to more than the number of cards in the deck.
So that’s selecting custom study, choosing tags, setting number of cards to more than number of cards in the deck.
To be honest, I’d prefer a button/option that does that automatically, i.e. a button that puts anki into a ‘study all the cards from any selected deck as much as you want until you decide to stop’ mode. Then when you stop anki can revert back to it’s intelligent selection mode until you toggle the button again.
It was clearly explained that Site Feedback is for meta discussion. You should’ve used the Suggestions tag here.
Anyways I know you did not ask for help here but I will still mention what most people would do instead of using that hypothetical button. So in Anki, you can deck options —> new card limit and set that to around a number you think is reasonably high. You can also do that in the Today limit which means if you need to study a higher number of cards you can just follow the above stated steps to change it easily. If you just put 9999 in there you can decide how many you want to do that day and can just leave the rest.
Also about custom study, I don’t use that thing but IIRC you don’t need tags there right? I mean don’t you just search the new cards if you need the new cards with deck:deckname is:new?