Interesting how many of you have elected to limited the number of grenades carried rather than to consider as taking up a "weapon slot" on your rig and carrying as many as you want by sacrificing some other weapon. If we're going by realistic weight, then by losing your sidearm along with its spare ammo and accessories, seems like you could increase your grenades carried by 3 for a deliberately grenade-heavy load...
Or just start carrying the grenade rifle, I guess, in place of my service rifle. I'm seeing accounts from Vietnam of carrying anywhere between 15 and 40 rounds of 40mm ammunition in various flavors, apparently depending on the strength of the individual and what their mission entailed. There's plenty of very serious pistols in the game to make up for not having a rifle. Would be interesting to re-create the buckshot rounds for the M79 too. Damage 10*27, spread *5? Pointless at long range, wipes out everything in front of you at short range.
It's more that we limit their number because we find it a bit silly to be toting 20-30 grenades around at once as that's just not done in actual usage. It also makes us consider the potential targets more carefully since we only have a small number of replacements on hand.
Note that this does not apply to Grenade Launchers or Rifles, for those the 20-30 grenades makes a lot more sense since they're actually 40mm shells. For the GMG it's generally about 5-7x that many as it goes through them at quite a rate, but said weapon does come with an ammo backpack to store them in.
Basically, we go for what would (more or less) be actual load-outs in order to keep things somewhat realistic.