Increase number with PHP (the first one is without a number)

Increase number with PHP (the first one is without a number)

I have a slide that uses caption (NivoSlider). So I'm using this code
<img src="<?php the_sub_field('sub_field_1'); ?>" alt=""
title="#htmlcaption" />
What I need is simple, but I really don't know how to do it:
The first image will be #htmlcaption
The second image will be #htmlcaption1
The third image will be #htmlcaption2
...
How can I do this with PHP?