Sorry if this may appear like a really dumb question, but I am new to RNAseq and feeling lost about some of the terminologies and calculations used in literature. 

I understand what's the difference between the principle behind paired-end and single-end reads, 
however I need to clarify the method to calculate number of reads in paired end vs. single end reads. 

For single end reads, I was told that I could calculate the no. of reads by: 
wc -l <name of FastQ file>, divided by 4. 

For paired end reads, because 2 files are generated (R1, R2), so is it correct that I calculate the number of reads by adding the read count derived from R1 and R2?

Thank you very much for your clarification. 
In paired-end sequencing its often less confusing to talk about fragment numbers rather than the more ambiguous read numbers. So count the number of reads in R1. That's the number of fragments sequenced (it's also the number of reads in R2). If you really prefer the words "reads", you could use "we sequenced XXX read pairs", so it's clear what's meant.