The unit counter on the galactic map is on the left of the screen under the planet's name and your Tech level. It's just a fraction with a numerator and denominator. The right number (after the backslash) is the maximum number of units you can have and the left number is the current number of units you have. If you click on a planet and see that the units you can build are no longer lit up or highlighted (they're faded) that means you've reached the maximum number of units. Also, how many planets are in your control determines the maximum number of units you can build. If you've had any of your planets conquered by another faction you may have had your cap lowered. See if that's it.
What faction are you playing? I'm playing Equal Footing too as the Empire and so far the Consortium is giving me tons of problems on the hard difficulty setting.
