How many switch games are available in Ps4 ?

Uh...

Well, anything by a major developer is likely on both consoles. It would be easier to find the Switch exclusive games and filter them out from a list of other games, because most titles have been already on the PS4 for a while.

Well for one thing, switch game cards are simple, put the game card in the system and you are playing the game. On ps4 you put the disc in the system, and you have to wait for it to install data which can take awhile. Depending on the game data size written on the game case label. Of course since is blu ray with 4K HD graphics and whatnot, it's not always gonna be as small as the switch games which currently hold 16GB on each cart (Until they get a bigger one.) Blu ray disc can hold about 50GB and the main reason for forced data install is to reduce load times.

For someone who has both ps4 and switch, I recommend buying switch game cards over anything. It will save you space and money on trying to get large SD cards. Use it for digital only games. I already filled up my HDD on ps4 with 1TB and currently looking for a USB HDD just to even get anymore games. I don't have much of a list for what you ask about the name of games that exist on both switch and ps4, but I can tell you what I do know currently.

Games I own or plan to own

• Puyo Puyo Tetris
• Mighty gunvolt burst (Gal gunvolt burst on ps4)
• Gal Gun 2
• BlazBlue Cross Tag Battle
• SNK Heroines Tag Team Frenzy
• Blade Strangers
• Street Fighter 30th Anniversary Collection

Games I don't have but know exist

• Resident Evil Revelations 1 & 2
• Fifa soccer 2018
• NBA 2K 18
• LA Noire
• Mega Man Legacy Collection 1 & 2
• Mega Man X Legacy Collection 1 & 2
• Naruto Ultimate Ninja Storm Trilogy
• Rocket League

There is plenty more but if i keep going, it will get too long

Also to end your last question "Why do I need a game for each system?"
A good reason, maybe if you play online, you have two sets of player base, one for ps4 and for switch, plenty of people do buy duplicate copies for each system for that reason. Probably more reasons but the only one that comes to mind at the moment.

Well the great thing that save money time and troubles for me is that i dont like online playing , except for Mario Kart with wii U , that has online free for a long time .
For the ps4 i bough a 4TB HDD so im saving all the backups that i can download for ps4 and switch there

Eventually it won't have online, and you will have to buy Mario kart 8 Deluxe with a paid subscription if you wanna continue playing that game online. So you may have something to think about.

Not really i played once every three month , and i have still many games i havent played yeat .