I only update when I feel like it.

swiper-slideが複数の時はデフォルト（ 左寄せ ）、ひとつの時のみセンター表示にしたい。

≪前提として以下のオプションを設定≫
slidesPerView: “auto”,
spaceBetween: 20,
※ Swiper 4.5.0 使用

Swiper
Swiper demo

HTML（Swiper / Multiple Slides Per View）

<div class="swiper-container">
   <div class="swiper-wrapper">
      <div class="swiper-slide"><img src="..." alt="..."></div>
      <div class="swiper-slide"><img src="..." alt="..."></div>
      <div class="swiper-slide"><img src="..." alt="..."></div>
      <div class="swiper-slide"><img src="..." alt="..."></div>
      <div class="swiper-slide"><img src="..." alt="..."></div>
    </div>
   <div class="swiper-pagination"></div>
</div>

<div class="swiper-container">
   <div class="swiper-wrapper">
      <div class="swiper-slide"><img src="..." alt="..."></div>
    </div>
   <div class="swiper-pagination"></div>
</div>

Javascript/jQuery

$( function () {
  $(".swiper-wrapper").each(function() {
    var swiperSlide = $(this).find('.swiper-slide');
    if(swiperSlide.length == 1) {
      $(this).addClass('justify-content-center');
      swiperSlide.addClass('mr-0');
    }
  });
});

CSS

.swiper-slide {
  width: 280px;
}
.swiper-slide img {
  max-width: 100%;
  height: auto;
}

.justify-content-center {
  -webkit-box-pack: center !important;
  -ms-flex-pack: center !important;
  justify-content: center !important;
}
.mr-0 {
  margin-right: 0 !important;
}