If you only have a battleforce for infantry, then to be run as an army Veterans are all you can run; you have enough models for a single platoon, but you'd need to have a second Troops unit and an
HQ unit before you can play. Right now, you can run them as 2 squads of veterans and 1 Company Command Squad with a heavy weapons team in each.
For the future, Somedude is right. There's a LOT of variation on how Guard can be played, but there is absolutely no way around the fact that you'll need a bunch of troops to do things. If you're planning to have more than one platoon's worth of infantry to make your army, you'll need another Battleforce MINIMUM and a Company Command Squad to have a legal army.
So, how many? That's up to you. The nice thing is with Guard a Veteran IS a Guardsman. Every Guardsman in the codex looks the same as the rest, they just have different guns on occasion. Why is this nice? Because you don't *have* to buy special boxes of "Veteran" infantry. If you have 2 Battleforces, you can build and paint all your models and then pull them out and put them on the table and you have the ability to arrange them into 2 platoons of standard Guardsmen OR you can make them into 5 units of Veteran squads OR 1 Platoon and 1 or 2 Veteran Squads. Not only is the army versatile, but so are the models!
If you wanted to jump in and start playing, though, you'll want more than just infantry to start with. This edition isn't particularly friendly to light infantry without armored support, at least not until you venture into the realm of an infantry horde which requires far, more than another Battleforce or two! That whole glorious image of waves of conscripts pouring fourth becomes rather dismal when you realize you'll want some 200 Guardsmen for a standard game! That's why, even if you want to have a lot of footsloggers, you'll probably want to get some other vehicular support. From 850 point games all the way up to Apocalypse, you'll almost always find effective use for a tank or Valkyries.
TL
R: Unless you don't want to play with your army at all until you've completely built an entire infantry horde that can stand on its own, look at getting some vehicles for support. I personally reccommend looking at whatever style you want as a "secondary" style and purchasing things that fit that with every infantry purchase. If you thought you'd also like to do, say, heavy metal, then consider looking at a tank whenever you feel like getting another infantry squad. Then you can mix and match and always have concrete staying power.
Out of curiosity, have you read through the codex much?