How do you balance acid base reactions?

You have posed an open-ended question, and it is hard to know for what you are looking. The general requirement is that both mass and charge be conserved, as they are in all chemical reactions.

In water, the generalized acid-base reaction is:

#H_3O^+ + HO^(-) rarr 2H_2O#

All acid base reactions are a variant on this theme. You have to observe the appropriate stoichiometry. For acids and bases that transfer 1 equiv of the protium or hydroxide ion this is fairly easy:

#underbrace(HCl(aq) + NaOH(aq))_"acid + base" rarr underbrace(NaCl(aq) + H_2O(l))_"salt + water"#

Of course this 1:1 stoichiometry mirrors that of the prior reaction. When we write (aq) besides a reagent we mean the #"aquated"# sodium chloride species, i.e. #Na^+# and #Cl^-#, a metal or non-metal ion surrounded by, or aquated by, several water molecules.

For reagents that deliver several equiv of acid or base, we change the stoichiometry appropriately:

#2NaOH(aq) + H_3PO_4(aq) rarr Na_2HPO_4(aq) + 2H_2O(l)#

How did I know that phosphoric acid reacts with just 2 equiv of sodium hydroxide? Well, I've done the experiment, and know the result.

So the moral:

#"Acid + base" rarr "salt + water"#

It is your job to find the stoichiometrically balanced equation.