You can define subfigures:

:::: {#fig:subfigures .figure .subfigures}
+:---------------------------:+:----------------------------:+:----------------------------:+
| ![a](fig1.png){#fig:subfig1 | ![b](fig2.png){#fig:subfig2  | ![c](fig3.png){width="100%"} |
| width="100%"}               | width="100%"}                |                              |
+-----------------------------+------------------------------+------------------------------+
| ![d](fig4.png){#fig:subfig4 | ![e](fig5.png){width="100%"} | ![f](fig6.png){#fig:subfig6  |
| width="100%"}               |                              | width="100%"}                |
+-----------------------------+------------------------------+------------------------------+
| ![g](fig7.png){#fig:subfig7 | ![h](fig8.png){width="100%"} | ![i](fig9.png){#fig:subfig9  |
| width="100%"}               |                              | width="100%"}                |
+-----------------------------+------------------------------+------------------------------+

::: caption
Figure 1: Caption. a --- 1, b --- 2, c --- 3, d --- 4, e --- 5, f --- 6,
g --- 7, h --- 8, i --- 9
:::
::::

:::: {#fig:subfigures2 .figure .subfigures}
+:--------------------------------------------------------------------:+
| ![a](fig1.png){#fig:subfig21}                                        |
+----------------------------------------------------------------------+
| ![b](fig2.png){#fig:subfig22}                                        |
+----------------------------------------------------------------------+
| ![c](fig3.png)                                                       |
+----------------------------------------------------------------------+
| ![d](fig4.png){#fig:subfig24}                                        |
+----------------------------------------------------------------------+
| ![e](fig5.png)                                                       |
+----------------------------------------------------------------------+
| ![f](fig6.png){#fig:subfig26}                                        |
+----------------------------------------------------------------------+
| ![g](fig7.png){#fig:subfig27}                                        |
+----------------------------------------------------------------------+
| ![h](fig8.png)                                                       |
+----------------------------------------------------------------------+
| ![i](fig9.png){#fig:subfig29}                                        |
+----------------------------------------------------------------------+

::: caption
Figure 2: Caption. a --- 1, b --- 2, c --- 3, d --- 4, e --- 5, f --- 6,
g --- 7, h --- 8, i --- 9
:::
::::

Figures themselves can be referenced fig. 2, as well as individual
subfigures: figs. 1 (a), 1 (b), 2 (i)