Psilocybe semilanceata are usually very small, a 1cm cap is a pretty big one for that species, usually about 30 average sized ones are about a gram dried, but they vary & they're stronger than cubensis by weight. So 300 is a very heavy dose, but not that far out of the normal range, like maybe 3x-6x a good strong dose, depending on how big the shrooms are. It'd be roughly like eating half an ounce of strong dried cubensis, or a bit more, maybe as much as an ounce if they're particularly big semilanceata. You still don't take that amount of mushrooms to have fun, you'll be on another planet, but it's probably not a blackout dose & not guaranteed hospitalisation, depending on the person & exactly how big the mushrooms are. 7 making you trip makes me think they're big semilanceata, so it's really maxing the dose. If 7 make you trip hard then stay well away from 300, you must be quite sensitive or they're totally different mushrooms.
If they're liberty caps, 7 big ones fresh out of the ground is just enough to get above threshold effects & start to trip, it's anything from about 0.1g to 0.5g dried, depending on their size, eaten fresh they can be really potent for their small size. 300 will give you an extremely intense trip & you will find it hard to separate the hallucinations from reality, that'd be over 10 grams dried weight, depending on size, 1cm is pretty big, so it might be closer to 20 grams. It's not going to send most people insane for more than a few hours. I'd suggest checking what mushrooms you have, if they're Psilocybe semilanceata, then work your way up with doses, don't just eat 7 & then eat 300 next time, that's dumb, you've no idea what a high dose of mushrooms will do to you, it's a totally different level to morning glory seeds or most other psychedelics. If 7 just barely make you trip, then 20 or 30, up to 50 maybe should be a good time, 300 is insanity. If you've had 50-100 of them before without freakouts then it's up to you, but it's not a sensible first dose & that kind of dose is something that would send somebody very experienced & comfortable with mushrooms completely wild. No other psychedelics prepare you for a really strong mushroom trip, they're different, I don't think any other drug can do anything as intense.
It's not a fixed amount of active chemicals in each mushroom either, larger mushrooms usually have much more per mushroom but slightly less by weight. 300 tiny liberty caps that have been dried & lost a little potency is way different to 300 big fresh ones, there could be 4 or 5 times the dose.
A lot of the people I know that got close to that kind of dose never took any mushrooms again.
If they're really much more than about 1cm in size, like a typical cubensis, then 300 is a truely ridiculous dose, I think you'd find it hard to eat that amount.
If they're actually some toxic mushroom rather than psylocibes then you will die. It'll likely be a horrible painful death too. It's extremely important to know what you're picking if you're collecting any kind of wild mushroom.
Check this guy out, he tries to explain what high dose mushrooms are like, from experience (fast forward about 15mins if you're impatient)...
http://vimeo.com/75940226 20-30g doses of dried cubensis (i'd estimate fresh semilanceata are twice that strength, not counting water content).