Well, if you mean...
How many Pokemon can you catch from the Nat. Dex? - Just all the Pokemon you can catch in FireRed, minus the trade evolutions. In the Nat. Dex, add basic Pokemon from R/S/E, G/S/C, and obviously R/B/Y. Don't forget that there are a few Legends, Rares, and Ancients that you can CATCH from the other games once you get the Nat. Dex. It'd take time to calculate.
In just the FireRed - 144-7 I think... No Gengar, Alakazam, Golem (I feel like I'm forgetting one?), and no game specific pokemon. So assuming you Include mystery gifts, you have Mew, Deoxys... (probably a few more I'm forgetting) as well that you can't get either.
Short Answer - A lot, it'll take time to figure out, but shouldn't be too hard. I have FireRed, and I have almost everything in the Nat. Dex without the trades, so I'll work on figuring it out.