One path to employer recognition of your union is to gather signatures on "union authorization" cards from enough of your coworkers to file for an election with the National Labor Relations Board (NLRB).
Legally only 30% of the workforce needs to sign these cards for the NLRB to initiate an election, BUT any campaign that wants to win must aim much higher than this percentage!
Every employer, without exception, will engage in a fierce anti-union campaign as soon as you file for an election with the NLRB. They will lie, intimidate, threaten, and even fire workers who are trying to organize and they'll accept the legal consequences of doing these illegal activities if it means they've prevented a union from forming.
Because of these tactics used by employers you have to anticipate that you'll lose a number of supporters for the union by the time workers can vote in an election. That is why you need to gather a super-majority of card signatures before filing for an election, so that you have the numbers and the power to overcome the boss' attack.