Dell Optiplex 790 (sff version) HOW MANY STORAGE CAN IT FIT?

unfortunately, the 790 sff can only accommodate one 3.5" hdd which is probably what you have, or up to two 2.5" ssd/hdd.  there is no space for one 3.5" hdd + one 2.5" ssd in the hard drive cage.

there are two ways to get around this.

(1) instead of 2.5" ssd, get a M.2 sata or nvme ssd and use a PCIex4 adapter for the ssd.  install the adapter like a card in PCIe x4 slot.  but the downside is 790 bios does not let you boot from a PCIe ssd.  if you clone hdd to the ssd, pc cannot boot from the ssd.  there is a way to get around this though, but it involves hacking the Dell bios and is meant for advanced user only.  if interested read this excellent write up by an MS MVP.

(2) remove the dvd drive, get a 2.5" dvd caddy adapter, install the 2.5" ssd in the caddy.  then install the caddy like a thin profile dvd drive.  it is better to connect the sata cable of caddy to either black or blue sata port on motherboard (not the slow white sata port).  you can clone hdd to the ssd and boot from it.

Is it okay if i just stick the ssd (I'm using a kingston a400)  with a double sided tape into the HD cage (the metal one)? I don't think I really have the budget right now and I usually dont get money.

Yes, you can if the 3.5 hdd is thin (20 mm) and 2.5 hdd is 7 mm.  This has been verified.

It is possible to install a 3.5+2.5 in the cage if the 3.5 hdd is thin (20 mm height) and the 2.5 hdd/ssd is 7 mm in z-height.   The total height would be 27 mm.

It is not possible to stack a 3.5+2.5 in the cage if the 3.5 hdd is thick (26 mm height).