20 Cards to 50 Cards
The frontier riverboat games actually forced poker to evolve into the game played today. Originally, the Persian-influenced game of As-Nas and early American poker used only a 20-card deck (Aces, Kings, Queens, Jacks, and Tens) played between four people. The cheating problem with only 20 cards in play was incredibly easy for riverboat mechanics to track, stack, and memorize every single card in the deck after just a few hands.

Riverboat Dealers
To combat this predictable cheating, riverboat dealers expanded the game to use the full 52-card English deck in the 1830s. This introduced the "Flush" (five cards of the same suit) and allowed more players to join, drastically increasing the mathematical complexity and making old-school tracking methods obsolete.