Each gate is enough for 10 slots (i.e., frequencies). If you have, say, 2 gates, 20% would mean 4 slots. You could create up to 4 full or half frequency routes. Best thing is to go to Gates --> View gates. Somewhere in there it'll show exactly how many slots you have free at each airport.
Regarding aircraft for W10, I don't have any data for models available in 1996. ATR 72 seem to be a popular, if somewhat pricey, choice. But since you're obviously just started, I'd suggest something smaller and, of course, cheaper. The Canadair CL-215T was a good choice in 1991. Maybe it still is. Embraer should also have a few nice models on offer, probably some variant of the EMB110.
Before buying, and particularly if you're still short on cash, have a look in the leasing market. Dirt cheap planes can do wonders for starter airlines.
