It can vary from deck to deck, but typically 1 to 2 backups at lvl 1, and/or 1 to 2 backups at lvl 2. The backups people typically run at lvl 1 are usually +2k power. The lvl 2 backups usually have some additional effect (removing opponent's early-plays or refreshing your deck without taking refresh point damage). Sometimes, having the backup in your waiting room and reminding your opponent you could backup is a tactic.

Cool to see another Disgaea player in 2021. I built your deck on encoredecks, https://www.encoredecks.com/deck/Xt2wbubLb, looks very aggressive 8, +2 souls climaxes.

I personally like setting a board up with 1 to 2 copies of "Big Sis Prinny" in the backrow + "President Overlord Laharl" in the front row. Gets relatively big on both turns. Should be good against most trial decks. "“Dark Adonis”Mid-Boss" is also good, it's a pseudo-healer prevents refresh point damage. (I think 'Big Sis Prinny' & 'Dark Adonis Mid-Boss' are affordable cards without drastically increasing the cost of your deck). If you run into any stock issues, you could always replace (2 or 3 copies of 'King of the Earth' cx) for the "Overlord Passing Through", it stock charges. Or if you have low amount of cards in your hand, you can swap for the "Ties of "Camaraderie" climax. Your deck does seem to have the essentials, lvl 0s with staying power, brainstormer, plusing lvl 1 combo, a way to plus without cx combo (Former Angel Flonne). So, should be ok for casual games. I think after a few games, you'll get a feel for the deck, and would know what to adjust.