Plastic Soldier Review has a long list of plastic figures for WW1, it shows there are 19 sets of Germans, 6 of French, 10 British, 5 British Dominion, 12 Russian, 5 Austro-Hungarian, 3 Italian, 4 American, 2 Ottoman, 1 Serbian, and 1 of pilots for all nations. That makes a total of 68. I have not included the sets made by MIR, HäT's reissues of Airfix's sets, or sets that have no figures, such as the LW/Evolution Russo-Balt armoured car, and some of the guns from HäT.
There are two sets available from Strelets that aren't on PSR's list; 6-inch Gun Mark VII, and 8-inch Howitzer Mark II. Both of these sets contain a sprue of figures for the gun crew, and there are actually quite a lot of figures. Bringing the total of plastic WW1 sets up to 70.
In addition to the 70, there are quite a few sets that will be released in the near or not-too-distant future.
Out of that 70, 26 are made by HäT, and if you include Coates and Shine as HäT (the boxes have "HäT" written on them, not Coates and Shine) the number goes up to 28. Strelets account for 22 sets (soon to be 24), with a large number of them being Russian and German cavalry. The remaining 20 are made up by Airfix (5), Emhar (5), Revell (2), Zvezda (2), Caesar (1), Pegasus (1), LW (1), Waterloo 1815 (1), Dark Dream Studio (1), and Co. Ma. (1).
Looking at those numbers you could be forgiven for thinking that there is a good selection of 1/72 plastic figures available for WW1. You'd be wrong, but you could be forgiven for it.