Trump's perception is probably widely shared. A variety of factors make it hard to compare current shooting regularity with the past, including disputed definitions of what constitutes a "mass shooting," the amount of media attention shootings get and the lack of comprehensive data from said past. But we can say with specificity that mass shootings are not uncommon.
In fact, using data from ShootingTracker.com, we can say that mass shootings — defined as incidents in which four or more people are shot — have happened hundreds of times over the last several years. In fact, during President Obama's second term, a Sunday-to-Saturday calendar week has not passed without a mass shooting incident.
Only once have seven days passed without a mass shooting and only once have eight days passed. Those are the longest spans — the latter happening in April of this year. Several times, six days have passed.
The most people shot in one day was on May 12, 2013, with separate mass shootings in Arizona, New Jersey, California and Louisiana.
Again, it's impossible to tell if Trump's assessment of the frequency of shootings is correct. But one thing is indisputable: They are frequent.