What is the difference between nfet_01v8_lvt and nfet_01v8_lvt_nf?

It's just a "convenience symbol" in xschem that displays the

nf

(number of fingers) parameter value.

Is there a difference in how total width is determined?

if possible use the nfet_01v8_lvt symbol. In this transistor the W parameter is the total width and nf is the number of fingers you draw on the layout. So if you look at the transistor layout each gate finger has a width of W/nf. In the nfet_01v8_lvt_nf symbol the W parameter is the single finger width, so the total width of the transistor is W * nf. It is just a different convention on the meaning of the W attribute of the xschem symbol. These *_nf symbols were requested by some designers. My suggestion: do not use _nf symbols. If you need to use them try to be consistent and do not mix the two different types in the same design.