Middle C on a 49-key piano keyboard:

RockJam (RJ549) 49-Key Portable Electric Keyboard Piano With Power Supply, Sheet Music Stand and Simply Piano App

Middle C on a 61-key piano keyboard:

RockJam 61-Key Electronic Keyboard Piano SuperKit with Stand, Stool, Headphones & Power Supply, Black – RJ561

Middle C on a 76-key piano keyboard:

Yamaha PSREW300 76-key Portable Keyboard With Knox Stand Knox Bench and Yamaha Power Supply

Middle C on a 88-key piano keyboard:

RockJam 88-Key Beginner Digital Piano with Full-Size Semi-Weighted Keys, Power Supply, Simply Piano App Content & Key Note Stickers

Piano Stickers for 49/61/ 76/88 Key Keyboards – Transparent and Removable