Its difficult to use exact numbers of combo's in game, one thing to note is its obviously easier if ranges are tighter, which is why HU poker is the toughest game to master. however, knowing how many combo's you have preflop allows you to make some decent assumptions on a given boards postflop. this is especially true for defending oop. for example if we are the bb defending vs a button open in a SRP and flop comes low - something like 357, we are gonna have a nightmare time calling down enough of the time vs a triple barrel vs an opponent who is aware of how the board is much better for them than us (they have all overpair combo's + offsuit combo's that smash that board, whilst our range is alot of offsuit 8-9-KT stuff. So once we just call flop we are somewhat capped and weak (pair+ draw at best most of the time) so in theory we would need to call down some hands like K5 (even when overcards come on the the turn and river) vs someone good who is opening wide enough preflop otb and using their advantage on that board well. The call down would depend on bet size, the smaller the bet the more hands we need to call.

lots of this is game theory based so not that important vs bad players, however one thing it has helped me do is make less hero call and know when to exploit more since when you break things into combo's you will find things out about calling betting that weren't clear before.

Having a good estimate of your opponents range and the number of combos of each of his hands can do amazing things for you. Here's a couple examples:

If you're the aggressor you can accurately estimate how often an opponent will fold or call if you bet or raise. Let's say you have an ace-high board, A92JKr, in a single raised pot and you're deciding if you should fire the 3rd barrel. You might make the assumption that villain never really has AQ+ in his range and will fold a lot of his baby aces to a 3rd bet. He will call any AT or better though. What you can do then is determine what % of his range he'll be folding or calling and then plan your bet and size accordingly.

You can also use it when villain is the aggressor. Maybe villain checkraises you on a 852fd flop in a spot with wide ranges. You can then make some assumptions that maybe he's checkraising only sets for value but he might have a bunch of flush draws and 76 and maybe some gutshots like 97 too. 3 sets are limited to 9 combos, but there could be a dozen flush draws, 16 combos of 76, etc and then you can play accordingly. It's one thing to know that all draws missed so he might be bluffing a lot, it's another to know he's probably bluffing approximately 70% of the time based on his range and aggressive tendencies.

Those are just a couple of examples, but essentially if you're a range and combo master you can estimate how often your opponent is bluffing, how often they have 1-pair hands (and then maybe you raise), how often they have 2-pair, sets, straights, etc.

It's also really nice to know how many combos of hands you have in your range, but I might need to make that in another post.

Thanks guys, been reading up and feeling a lot clearer.