H2SO4 is a strong acid which means it will immediately dissociate in water to yield H+ and HSO4-.  It has a terrific affinity for water as we can see in common demonstrations such as sulfuric acid dehydration of sugar. Very exothermic, which is why we always add acid to water and not vice-versa.

In theory, an equimolar amount of water will fully consume the H2SO4 - there will be none left and we'd have H3O+ and HSO4-. However, while we say "completely dissociates" we are being a bit glib. The Ka1 is on the order of 10^3, so that means the equilibrium lies strongly to the right, so this is an oversimplification. Regardless, the temperature spike on that would be nuts and vaporize some of the water unless this was done in a controlled fashion. So we need more water than equimolar.  But there's a lot more to this because we have to think about the enthalpy of mixing (heat of dilution). I could throw up a bunch of calculations but a more pragmatic approach is to ask if any diluted solutions of sulfuric acid are considered water reactive.