The card type ratios vary greatly with the deck strategy, but there are a few rules of thumb that are generally true, unless you are building a specific type of deck that requires it to be different (such as Heroless/Herolite, or other non-standard deck builds).
In a 50 card deck, you should have the following:
7-8 Lost Souls (8 if you use Hopper, which is pretty standard in 50 card decks).
1-3 Sites--1 Access site if you don't have other reliable ways of site access, then 0-2 Sites that help your defense or part of your strategy.
3-6 Artifacts (including Covenants and Curses)--The number and choice will depend greatly on deck strategy, and certain builds may require less than 3, but in most cases I'd say 3 is the practical minimum.
0-2 Fortresses--Fortresses aren't required for every strategy, but most decks find at least 1-2 of them to be useful.
7 Dominants--The 7 you choose is up to personal preference and deck strategy, but there is little reason you would ever want to use fewer than 7, unless you have some sort of combo deck where every card has a specific purpose.
7-12 Heroes--A good balanced deck has 7-8 Heroes, whereas an offense-heavy build will use 10-12. You should only use more than 12 Heroes in a 50 card deck if you use little to no enhancements, and thus depend on your Hero abilities to win. You should always have at least as many Heroes a good enhancements, except in rare cases.
3-8 Good Enhancements--I'd suggest staying in the 6-8 range, unless you use a Heroless/Herolite build, or you have a 13+ Hero offense as mentioned above. As before, never have more good enhancements than Heroes, except in rare cases.
4-8 Evil Characters--Decks used to be competitive with fewer than 4 Evil Characters, but in recent sets, 6 is a practical minimum for a competitive deck, even offensive heavy. As with Heroes, you should always have at least as many ECs as EEs, and unless you are building a defensive heavy deck, you should also have fewer (or the same number) of ECs than Heroes.
3-8 Evil Enhancements--For EEs, I'd suggest staying in the 4-6 range, again unless you are building defensive heavy.
DAEs, or dual-enhancements (the good/evil ones from the last set) will typically count as one of the other two types, and it may depend on what you will most often use it for.
If I were to build a 50 card deck, it would likely include:
8 Lost Souls
2 Sites
4 Artifacts/Covenants/Curses
7 Dominants
2 Fortresses
9 Heroes
7 GEs
6 ECs
5 EEs