Does the number of players I offer have to match the number of players I'm trying to acquire?

No.  The only rule with regard to the number of players involved is that each team's roster may have a maximum of 28 players. 

And you can trade as many players as you want. The only requirement is that once the deal is over, the roster for both you and your trading partner can have no more than 28 players, total.

So, 2 for 1 deals are possible, as well as 3 for 2, or even 3 for 1, whatever you and your partner determine is a 'doable' trade.  However, the team receiving extra players must make sure their roster can accommodate the additions.  If that team's roster is already maxed at 28, but they are giving two players, receiving 3, that would be over the limit.  This team will then need to choose a player on the current roster to 'release.'  When the trade is processed, all players involved in the deal change teams, and the released player goes into the FA pool.