Spending on a wheel is called RNG, you might get it, you probably won't.

Saving for 20,000 coupons is recommended because that is how much most of the high tier ninjas cost (6 paths Naruto, edo itachi, Konan angel of God, Minato edo tensei, sasuke rinnegan). Best events to use those coupons are those that give rewards besides the ninja you are spending towards. So events like lucky stars and lucky dice, and to a lesser degree mission mobilization. There is also an event called fukurokumaru deals that gives a rebate for coupon spending and usually has a good ninja in it.

Ninjas to go for, 6 paths obviously, edo minato, are some good ones that are at 20k. Edo deidera is a good one if you go for 180 fragments for his skill breaks (80 fragments to recruit, 100 to 4 star and then use advanced vials) but he costs around 35k all together.

As a f2p, never do RNG event for the purpose of getting the 80 frags as you aren't guaranteed.

People save up to 20,000 coupons is for when a certain event comes called Fukurokumaru's Deals for the month. The event will give you rewards based on how many coupons/ingots you spent. Pay attention by reading the event updates for each week as to when it comes. On average, 20,000 is the maxmium amount needed to get the rewards. Rewards range from ninjas and items. Spending your coupons during the when Fukuro deals is available is solely up to what other events is available for that week. So people would either spend on ninjas, clothing, items, or a combination depending where they are.

Yellow scrolls, seal scrolls? Best to save up to 300 and use them in the Kage, Gnw, or the jinchuriki temporary treasures.

Shisui is still a good ninja with a good mystery, good barrier, and dodge. He also costs around 10k coupons

Madara founder is a super rare from GNW treasure (about 800-1200 seal scrolls is needed) that pairs well with him.

Killer bee is a rare from kage treasure and the pull rate is about 60 seal scrolls and you have a 1/8 chance to get him.

Edo roshi is from the limited jin treasures and requires about 220-240 with a 50% chance of getting him. Keep in mind, he most likely appears in events for coupons/ingot sale.

When LolOasis says save up to 300 seal scrolls, he meant to save up for the seal scroll rebate as you will get back seal scrolls back depending on the amount you used.