I'm running one with 5e right now in a modified version of the LotFP's Weird New World and I haven't run into an issue around numbers of encounters yet... Now that said I do make use of wandering monsters in my hexcrawl, which helps to increase the number of combats (I've been using a modified version of the one in the Starter Set adventure).
The other thing is that I have populated many of the hexes with ruins from an earlier civilization that once ruled the "Far North" so a single site can, and usually does, have multiple combatants, traps, hazards, etc. when it is being explored. Now admittedly I use the encounter builder/adventure day xp guidelines as just that... guidelines. In other words I've taken areas in the Weird New World and designated an XP range for them then populated within the area with an eye to making sure they fall within that particular range... though again it's not a hard and fast rule. the PC's are of course free to explore wherever they desire.
Finally, at least as far as I am concerned, the number of encounters per day hasn't really been the most important thing for my hexcrawl. I think keeping the tension up and forefront as well as a sense of the unknown and dangerous just beyond where they have explored is what, at least IMO, makes a hexcrawl fun.