Yeah, it's only one per year.
When a player is added to a 40-man roster for the first time, the major league team is permitted three optional assignments of his contract, or three "option years." This gives them the option to assign that player to the minor leagues without requiring him to clear waivers. For each season thereafter in which the player is assigned to a minor league team, one option is used up.
When a player is out of options, he can still be assigned to the minor leagues, but first he must clear waivers.
A player can receive a fourth option if he has less than five seasons of pro experience. Draftees who immediately sign a major league contract will qualify unless they reach the majors quickly and stick there. Otherwise, they'll have their three options exhausted after their first three years in pro ball. A season is defined as any year in which the player spends 90 days on the active list. Short-season and Rookie leagues don't last 90 calendar days, so a player assigned to those leagues for an entire year won't accrue a season of pro experience. Also if a player has a long-term injury, he usually won't be credited for a season that year. (The exception is if he goes on the disabled list after spending 60 days on an active list, in which case the DL time counts as service time.)