As it stands, there are 5 different ways to select units in Strike Tactics. 

1. Single click a unit

2. Double click a unit to select all units of that type within the camera

3. Click the icon at the top-right of the screen to select all units of that type within the entire game world

4. Click drag to select multiple units within a rectangular area 

5. Shift click a unit and it will be selected along with any units currently selected

1, 2, 4 and 5 are pretty standard in RTS games and really need no explanation. I've implemented #3 because I personally found it useful. Certain units are good against certain units - why shouldn't you be able to select only a certain type of unit? 

Control groups

As-is standard in modern RTS games, control groups are a thing. Hit CTRL-1 to create a selection group which is active when you hit 1. 

Once a unit is selected, its icon, along with health bar, will appear in the lower-middle of the screen. I have decided to do away with the single unit profile box, you see in a lot of RTS games. You know, the box that will sometimes show stats and a large picture. I've deemed this little UI element to be superfluous and a carry over from 90s RTS UI design. Unit stats will be available in-game, except it will be through the production menu (i.e., you will see a summary of stats for the unit when you hover over the icon in which you click to build the unit) .

Currently, there is no cap on the # of units you can select. RTS games used to implement limits because finding 100 paths at the same time is resource-intensive, and can hault the game engine, depending on various factors (this was a problem back then, when you have 128MB of ram and 366MHZ processors). Thankfully, modern browser computing isn't quite as limited as peak 90s computing. At least my particular pathfinding implementation is efficient enough for this not to be an issue.