There is a lot you can do with 40,000 troops, but conquering Ukraine is not one of them.
This article does not state that, but it does lay out an overly frightful scenario: 40000 Russian Troops are Preparing for War in Crimea
The scenario they lay out towards the end of the article is “…a full-scale Russian military offensive likely would aim to seize key military-industrial area such as tank plant at Kharkiv, the missile factory at Dnepropetrovsk, the shipyard at Mykolyev, and the port of Odessa. Russian forces also could drive into Ukraine from the northeast to the outskirts of Kiev and place the capital within artillery range in a bid to force a change of government.”
As the article notes: “Still, Russia does not appear to have all the forces in place for a major military operation…”
So how big is the Ukrainian Army? Well, according to Wikipedia they had 250,000 active armed forces personnel in March 2016. Their source was a Reuters article: Reuters
According to Wikipedia the ground forces were 204,000 in 2009 (Air Force was 36,300 in 2009 and Navy was 6,500 in August 2015). In October 2013 President Yanukovich ended conscription (both the Ukrainian and Russian armies rely heavily on conscripts), but it was reinstated in 2014 after Russia intervened in Ukraine. As Wikipedia notes in a dated posting: “Due to the reintroduction of conscription, and partial mobilization, Ukraine’s armed forces is expected to nearly double from approximately 130,000 personnel in December 2014 to approximately 250,000 personnel in 2015.”
Anyhow, not the best and most current data, but safe to say that the Ukrainians probably have 200,000 or more ground troops available. Numbers do matter, and while 40,000 Russian troops are certainly a threat to Ukrainian security and stability, it is not a force that ready to march to the outskirts of Kiev or take Odessa. It would require a very serious mobilization effort on the part of Russia to do that.