I would highly recommend you read a couple of blog posts (1 and 2) from Daniel Wallace at Parchment and Pen on the topic of textual criticism. Few are better equipped to address the issue than Wallace. He is very involved in the study of the Greek manuscripts, and very knowledgeable in the field of textual criticism.

Wallace notes that many Christians falsely define what a textual variant is, and misunderstand how textual critics have arrived at the conclusion that the NT text contains 300-400K variants. It is commonly believed that this number is so large because any given textual variant, when it appears in multiple manuscripts, is counted multiple times. So if a single variant in Romans 5:1 appears in 10 different manuscripts, it is counted as 10 variants. Not so. Read Wallace’s posts to uncover the truth about how textual variants are counted.