There are blessings that we say when we hear certain things (good news, bad news) and blessings said on seeing certain sights (eg a wise person, a powerful ruler or a rainbow).
Most common among them are probably the blessings for hearing thunder and seeing lightning. On hearing thunder, we say shekocho ugevurato maleh olam, “His strength and might fills the world”.
Oseh maaseh bereishit, “who makes the work of creation”, is what we say for seeing lightning. The sudden flash of light evokes God’s first act of creation, “Let there be light”.
If the thunder and lightning are simultaneous (or seem to be — I know — they are never exactly simultaneous), then you only say one blessing, whichever one you like. You say each blessing once per storm, no matter how many lightning flashes and thunder claps there are.