How many spaces for tab character(\t)?

A tab character should advance to the next tab stop. Historically tab stops were every 8th character, although smaller values are in common use today and most editors can be configured.

I would expect your output to look like the following:

123456789
a       b
        c

The algorithm is to start a column count at zero, then increment it for each character output. When you get to a tab, output n-(c%n) spaces where c is the column number (zero based) and n is the tab spacing.

Imagine a ruler with tab stops every 8 spaces. A tab character will align text to the next tab stop.

                                0       8       16      24      32      40
                                |.......|.......|.......|.......|.......|
printf("\tbar\n");              \t      bar
printf("foo\tbar\n");           foo\t   bar
printf("longerfoo\tbar");       longerfoo\t     bar

To calculate where the next tab stop is, take the current column.

nextTabStop = (column + 8) / 8 * 8

The / 8 * 8 part effectively truncates the result to the nearest multiple of 8. For example, if you're at column 11, then (11 + 8) is 19 and 19 / 8 is 2, and 2 * 8 is 16. So the next tab stop from column 11 is at column 16.

In a text editor you may configure tab stops to smaller intervals, like every 4 spaces. If you're simulating what tabs look like at a terminal you should stick with 8 spaces per tab.

A Tab character shifts over to the next tab stop. By default, there is one every 8 spaces. But in most shells you can easily edit it to be whatever number of spaces you want (profile preferences in linux, set tabstop in vim).