In the original Paper Mario, Mario can carry a maximum of 999 coins. The Nintendo 64 instruction manual states this limit directly, making 999 the canonical answer rather than a gameplay estimate.
Coins are the game's main currency. Mario earns them from blocks, defeated enemies, found objects, and various activities, then spends them on items, badges, and services. Because useful purchases appear throughout the adventure, the coin counter is a practical part of exploration rather than a decorative statistic.
The limit is easy to confuse with other Mario games. Super Mario 64 also uses a 999-coin in-level cap, while later Paper Mario releases changed their economies and storage limits. The Nintendo Switch remake of the original Paper Mario raises the maximum to 9,999 coins, but that does not apply to the Nintendo 64 version.
Another related limit concerns how coins are acquired: some battles and objects can provide substantial rewards, but once Mario reaches 999, additional coins cannot increase his total. Thus, 1,000 is not a valid carried balance in the original game.