They said that "the Lamb" in Revalation 14:1 was Jesus and and since there is only one "Lamb of God" the 144,000 standing with "him" must be a true number, not symbolic.
You're reading a number into the text when there is none mentioned. You can infer a number from the singular article, but there is no parallelism between numbers. However the "Lamb" and the "144,000" are both figurative expressions. Here is a rather goofy instance of this weak argument:
***w60 6/15 p. 384 Questions from Readers ***
Further, at Revelation 14:1, 3, we read of Jesus Christ as standing upon the mount Zion together with 144,000 others. The 144,000 would logically be as literal as the 1, making a total of 144,001 standing upon the mount. Besides, why give such a large and specific number if it does not impart information?
Aside from addition, there is also subtraction of the apostles, resulting in the seemingly precise number 143,989 of anointed Christians:
***w61 3/15 pp. 165-166 Should You Partake of the Lord?s Evening Meal? ***
Other scriptures show that sharing this glory with Jesus and his eleven faithful apostles will be 143,989 others, for a total of 144,001.