It does depend on the card set, but to go into sufficient detail as to be useful:
(1) How many Witches should you have? You're right to be leery of stuffing your deck with too many. But how many is too many depends on what other cards are available. If there are Village-type cards available, you can buy some of those, which will allow you to support more Witches. Play a Village, and you can then play two Witches on the same turn, so you can afford to purchase more Witches.
But without a source of +2 Actions, extra Witches that come up in one hand are dead cards, so you will want to buy fewer Witches. It's also worth noting that the Curses might very well run out before the Witches do, so denying Witches to your opponents may backfire big time: If you're the one with more Witches, your hand is the one weakened more when the Curses run out.
(2) How many of some other card should you have? Some cards, usually the terminal ones, you only want one, maybe two of. As powerful as Chapel is, you rarely need more than one. But others work better when you get as many of them as you can. Minion and Conspirator, for example, are both fine on their own, but to unlock their real power, you need as many of them as you can get and not much else.
(3) Rather than thinking in terms of how much total treasure you have, think in terms of your average treasure value. If the average value of your treasure cards is, say, $1.5, then buying a silver will increase the average wealth available to you on any given turn. But if it's $2.5, then buying a silver actually makes you poorer, if you think about it. Imagine if all your deck consisted of was 5 golds. You'll get $15 to spend next turn. Say you buy another gold. Now you'll get $15 next turn as well. But if you bought a silver, you will probably only have $14 then. You've increased your OVERALL wealth, but hurt the stat that matters: how much wealth will the average hand draw?
Of course there is a second variable here: the ratio of treasure cards to other cards. If rampant Witching has left you with a deck full of curses, then buying a silver -- and maybe even supplementing them with a few coppers -- might help loosen things up a bit. On the other hand, if your deck is bloated that badly, you won't get to use those treasure purchases very much anyway, if at all. It's a complicated balancing act that others here have a better sense for than I do. But these are the sorts of considerations you can take into account.
I'll leave your last question, about trashing treasure on $7 hands, to someone with more insight.