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.

Shisui is more so at the 10,000 mark now a days. You should focus on getting a carry ninja like LolOasis mention and not a secondary hitter ninja like shisui.

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.