Things are a bit tough for squishy models like gaunts in the current game as it is so deadly with shooting If you want to take some Hormagaunts you need to suitably cover numbers for the "light infantry" bracket or they'll get blasted off the board easily. I think squads of 10 are viable if you're taking multiple squads to spread numbers out. A single large squad seems a bad idea to me as you might be able to give them suitable buffs but they'll make for an easy target for enemy small arms. I reckon such a larger squad needs supporting smaller squads too, but I'm not sure how this could look for decent results.
The numbers game helps them get into combat where they need to be as they'll drop like flies on the way in so you need to weigh cost and reward for how you get them there - do you take minor detours for cover benefits or go straight for gold? Aside from not having guns their main use is locking things in to combat as they don't hit particularly hard so aren't much of a threat to anything beyond other light infantry. It's good if you can get a juicy target locked in so it can't use it's guns to good effective but that's not always possible, so they're usually the anvil for something more capable.
Tyranids are quite diverse with how you can play, so I'll be interested to hear other experiences with using Hormagaunts. I like to use them in number where the cost of upgrades is less optimal but they certainly kept the enemy busy