One more tuba, one more percussionist.
Most modern band music has eleventy percussion parts. If you only have four a year and you lose two, then you can't cover everything, or you have to pull winds to play the parts. And with three tubas - all it takes is one zealous football coach and you have one or no tubas left. Always, always, always over-recruit tuba if you have the horns to put in their hands.