how many platforms are there for mobile app development?

Xamarin.
PhoneGap.
Sencha.
Appcelerator.
iFactr.
Kony.
AlphaAnywhere.
Redhat.

There are tons of them... when in doubt look at wikipedia: https://en.wikipedia.org/wiki/Mobile_app_development

Nowadays Mobile Applications are very essential for online business.

There are several Mobile App Development platforms to develop mobile apps.
Here I would mention some of the important mobile app development platforms.

Configure.it
Xamarin
PhoneGap
Appcelerator
Telerik
Appery.io
AppMakr
AppsBar

These are all the good Mobile app development platforms that every mobile app developer should try.

There are many mobile app development platforms are available at online where you can design an app like PhoneGap, Xamarin, Appcelerator, Telerik

The purpose of such platforms is to save time and effort on the part of the developer.Developers can reuse the code and can design apps that can work efficiently on multiple platforms,including Android, iOS, Windows, and many more.

Mobile applications are no longer a luxury,but have become a necessity of day to day life.One of the biggest challenges that app developers faced before was creating applications that work well on multiple platforms.The latest app code supports the concept of write once,run anywhere (WORA).The developer writes code once on one platform, and runs it on multiple platforms.

There are a number of Mobile app development tools that can create and develop application:
1. Xamarin
2. kotlin
3. spring
4. swift
5. PhoneGap
6. NativeScript
7. React Native
8. Ionic Framework
9. Flutter
10. Sencha Touch
11. Apache Weex
12. Framework 7
13.Codename One
14.Monocross
15. RhoMobile

Mobile application development is more important to the enterprise than ever before.Development organization are increasingly looking for a mobile application development plateform that can support their needs for both current and future projects.These tools are useful because they decrease costs and increase the speed at which apps are developed.

PhoneGap and Flutter is one my favorite choice. I always recommend these two.

There are many Mobile App Development platforms to develop mobile apps so i am sharing few of them
1.Sencha.
2.Kony.
3.AppMakr
4.Redhat.
5.Appcelerator
6.Appcelerator.
7.Xamarin
8.iFactr.
9.Flutter
and many more...

Top Mobile Platforms:

Codename One
PhoneGap
Appcelerator
Sencha Touch
Monocross
Kony Mobile Platform
NativeScript
RhoMobile

There are number of platforms available for mobile app development. It is depend on the android app development or Iphone app development...

here i mention some topic which help you in mobile app development
Jazzy
Dash
SwiftDoc.org
Flutter
Sencha Touch
Apache Weex
Framework 7
Codename One
Monocross
RhoMobile

This all depends on what you want your app to do, what you want it to integrate with and how much time you have to build and maintain it.

Essentially, you have three options:

Native mobile apps, which are developed in a programming language native to the device and operating system (android or iOS), and require one specific app to be created for one target platform.

Cross-platform mobile apps, which are developed using an intermediate language, such as Javascript, that is not native to the device's operating system. This means that some, or all, of this code can be shared across target platforms - for instance, across both iOS and Android.

HTML5 hybrid mobile apps, which are cross-platform apps but render the user interface using an embedded web browser, leveraging HTML, CSS and Javascript.

1. Kalipso Studio
2. Appery
3. Bizness Apps
4. Appy Pie
5.GoodBarber
6. Appsmakerstore
7. Mobincube
8. Shoutem
9. MobileRoadie
10. AppInstitute

Best mobile app development platform such as -
PhoneGap
Xamarin
Ionic
Intel XDK
Accelerator Titanium

Mobile application development companies develop mobile applications on different platforms in view of their client require. However, the significant platforms are Android, iOS, Cross-platform and web platform and beneath are very much utilized as a part of past years. iOS: You can use Xcode and Swift.

I can see that you already get a list of different mobile app development platforms to choose from.
I suggest you to hire a mobile app development agency, talk to them about your business or profession for which you are willing to develop an application and let them suggest you the best platform accordingly.

There is a number of platforms for the Android app development:
Configure.it
Xamarin
PhoneGap
Appcelerator
Telerik
Appery.io
AppMakr
AppsBar
for more, you can check on wikipedia