When dealing with large integers or real numbers in 19.x / 19 R5 or older, the highest representation in an integer would be 12 significant digits. However, 19 R6 now uses double-precision binary which handles up to 17 significant digits of precision.

19.x / 19 R5 and older:

Note that the last "3" digit in $testString was rounded to 4 when converted to a num.

19 R6 and newer:

Note that all digits remain intact when converted to a num.

Be sure to use 19 R6 and newer if the work deals with this level of precision.