not all music syncs to my iPod

Show your iTunes Music library. In the sidebar (left side of iTunes window), under Library, click Songs. To the right, your music library appears as a plain song list, with columns. Do you see a narrow column with a cloud symbol in its heading?

If you do not, right-click the heading row (to show a menu of all columns), and select iCloud Download (so it has a checkmark next to it). iCloud Download is the name of column with cloud symbol in heading. You can move it (left or right) on the song list, by dragging by its heading. I like putting it immediately to the left of song Name column.

In that cloud column, songs that are only in iCloud have a cloud with down-arrow symbol (the download cloud). You can click that symbol to download the song. But it's be "a bit" tedious to do that thousands of times... 🙂

So, click heading of the cloud column, to sort song list by that column. Now, all songs with download cloud are grouped together on song list. Select them all at the same time. To do this, click first song (with download cloud), scroll song list, press and hold down Shift key, and click last song (with download cloud). The two songs you clicked and all songs in between are selected at the same time. Right-click selection, and Download. With a few thousand songs, it may take a while...

To be clear, iTunes syncing is generally in one direction only, from iTunes library to iPod. The only songs that iTunes can transfer from iPod to computer are purchased songs from iTunes Store (the "about 650 songs"). The rest of those "about 6000 tracks" were erased from iPod, when you set up automatic syncing (the Sync Music setting) using a new iTunes library that is different from the previous iTunes library.

Then when I tried to sync the iPod with the full library only a limited number of songs synced to the iPod and not all playlist transferred.

About how may songs is "a limited number of songs," compared to the total number of songs in your current iTunes library?

Find one of the songs that does not currently sync to iPod, in your iTunes music library. Right-click it and Get Info. On Info window File tab, find location. Does location show a file path to where that song's file is stored on your computer's drive, or does it say iCloud.

If it says iCloud, that song is currently accessed from iCloud. If you subscribe to Apple Music or iTunes Match, that would be your iCloud Music Library. Purchased songs are similarly assessed for streaming from iTunes Store (if you don't have an iCloud Music Library). Song files must be stored locally, on your computer's drive for direct syncing. So, it's possible that "some 1494" songs are, and the rest are not.

If that's not the problem (all of your song files are stored locally), please post back with more details about your situation.