Based on this Topps collection guide by @samsamsamsmsms (please correct me if I’m wrong Sam), there are these variations for the regular #25 Pikachu:
- Non-Holo, with blue logo, circular Nintendo logo, and black Pokémon name at the back
- Smooth Silver Holo, with blue logo, circular Nintendo logo, and black Pokémon name at the back
- Non-Holo, with blue logo, oval Nintendo logo, and red Pokémon name at the back
- Smooth Silver Holo, with blue logo, oval Nintendo logo, and red Pokémon name at the back
- Smooth Rainbow Holo, with blue logo, oval Nintendo logo, and red Pokémon name at the back
- Non-Holo, with black logo at the back
- Smooth Silver Holo, with black logo at the back
- Rays Silver Holo, with black logo at the back
- Rays Rainbow Holo, with black logo at the back
- Non-Holo, with green logo at the back
- Flames/Teardrops Silver Holo, with black logo at the back
- Flames/Teardrops Rainbow Holo, with black logo at the back
- Non-Holo, with red logo at the back
- Cross-hears (Pokéballs) Silver Holo, with black logo at the back
- Cross-hears (Pokéballs) Rainbow Holo, with black logo at the back
These for the TV2 Pikachu:
- Non-Holo, with blue logo, circular Nintendo logo, and black Pokémon name at the back
- Smooth Silver Holo, with blue logo, circular Nintendo logo, and black Pokémon name at the back
- Non-Holo, with blue logo, oval Nintendo logo, and red Pokémon name at the back
- Smooth Silver Holo, with blue logo, oval Nintendo logo, and red Pokémon name at the back
- Smooth Rainbow Holo, with blue logo, oval Nintendo logo, and red Pokémon name at the back
- Non-Holo, with black logo at the back
- Smooth Silver Holo, with black logo at the back
- Sandpaper Silver Holo, with black logo at the back
- Sandpaper Rainbow Holo, with black logo at the back
- Non-Holo, with green logo at the back
- Squares/Rectangles Silver Holo, with black logo at the back
- Squares/Rectangles Rainbow Holo, with black logo at the back
- Non-Holo, with red logo at the back
- Dents Silver Holo, with black logo at the back
- Dents Rainbow Holo, with black logo at the back
And these for the Chrome Pikachu:
[* Standard Holo
- Spectre Holo
- Sparkle Holo
- Tekno Holo](https://i.imgur.com/GHtaNm9.jpg)
There are of course also loads of other Topps Pikachu cards: in the Topps Advanced sets (i.e. this one or this one); the clear Flying Pikachu & Surfing Pikachu; the Chrome Topps Jumbo Pikachu and Flying Pikachu; etc. As well as loads of cards featuring Pikachu, like those from the first Pokémon Movie.
EDIT: I now see that the Chrome and regular Pikachu are the same artwork, so ignore the section about the TV2 one with a different artwork.
Greetz,
Quuador