From AnalogLogger.ino example (SdFat examples), increases the "..00.CSV" by one each time the sdcard starts:
..
// create a new file in root, the current working directory
char name[] = "LOGGER00.CSV";
for (uint8_t i = 0; i < 100; i++) {
name[6] = i/10 + '0';
name[7] = i%10 + '0';
if (sd.exists(name)) continue;
logfile.open(name);
break;
}
if (!logfile.is_open()) error("file.open");
cout << pstr("Logging to: ") << name << endl;
..