How many feeder fish will my slider turtle eat per week?

I need to know how often and how many feeder fish to buy. I'm preparing to purchase a red-eared slider. Also, can I use pebbles from the river/creek for the substrate and an outside slanted rock for the basking area? I'm trying to go as natural (aka cheap) as possible. My slider will be in a 55 gallon tank with a heater, thermometer, filter, lid with lighting and also a UVB lamp over the basking area. But I would like to use a 'natural' substrate and beach. Thanks for any ideas/suggestions!