There is a simple reason your COUNTA returned 501 instead of 500, and it has nothing to do with your formula. You started counting at zero.
The user who posted this question labeled cell A2 with the number 0, then filled down to cell A502 labeled 500. That is 501 cells with values. COUNTA counts every non-empty cell in a range. It does not care about labels or what number you think should be at the end. It counts what is there. When your first data point is zero and your last is 500, you have 501 entries. The formula is working exactly as designed. The mistake is thinking the label "500" means there are 500 items. It means the last item is 500, which is the 501st item if you started at zero.
This is a common trap. Spreadsheets are literal. They do not infer intent. If you want exactly 500 values, your first cell should be 1, not 0. Or you can use `=SEQUENCE(500)` in a single cell and let the function generate exactly 500 rows starting from 1. That brings us to the second part of the question: filling cells faster without a subscription. The user mentioned struggling with SEQUENCE and lacking access to paid fill tools. The free version of Google Sheets supports SEQUENCE. Type `=SEQUENCE(500)` in cell A2. It will populate 500 rows with numbers 1 through 500 instantly. No dragging, no subscription, no error. If you need to start at zero, use `=SEQUENCE(500,1,0)`, that gives you 0 through 499, which is 500 values.
The deeper point here is that modern spreadsheet tools already solve these frustrations, but only if you know where to look. COUNTA is a counting function, not a mind reader. SEQUENCE is a generation function, not a mystery. The barrier is not the tool, it is the gap between what users expect and what the function actually does. That gap is exactly what AI-native spreadsheets are designed to close. Imagine asking your spreadsheet "fill 500 numbers starting from zero" and having it understand you without needing to remember the exact syntax. That future is already being built. For now, learn the function. It costs nothing and saves hours.