Supernova remnants are believed to be promising sources of Galactic cosmic rays. One of the principal questions is whether they are accelerating particles up to the maximum energy of Galactic cosmic rays ($\sim$PeV). In this work, we systematically modeled the gamma-ray spectra of 38 remnants to constrain the particle-acceleration parameters of freshly accelerated particles with the contribution of released particles remaining nearby taken into account. The average maximum energy during lifetime was found to be $< 20$ TeV ($t_{\rm M}/\text{1 kyr})^{-0.8\pm0.2}$ with $t_{\rm M}$ being the age at the maximum, which reaches PeV if $t_{\rm M} < 10$ yr. The maximum energies even at similar ages were fount to have a variety of 1--2 dex from object to object. In order to further investigate such environmental dependences, we then studied supernova remnant RCW 86, where shock velocities are measurable and both thermal and non-thermal X-rays are spatially mixed. The non-thermal X-ray parameters showed clear correlations with plasma density, not with shock velocity, indicating that particle acceleration physics in this remnant is controlled by the ambient density. This can be qualitatively understood with a magnetic-field amplification via shock-cloud interactions.