I enabled labels for a streets layer in my QGIS 3.6.0 project via Properties > Labels > Single labels. The following screenshot shows an example of such labeling:
How can I hide individual labels if the there are too many as in the example?
An easy method is to add a field to your layer, for example you can name it
show_lbl. Make it integer type.
For the features where you want to show labels (I assume most of them), fill it up with the value
For the features where you do not want to show labels, fill it up with the value
Then in the label dialog, where you configure the label display options, add a filter like
"show_lbl" >= 1.
The feature labels should be displayed/hidden to your liking.
Under Layer properties > Labels > Placement > Repeat you can enter "No repeat" It's case sensitive so just like that. If the Label is generated from a Attribute and you have multiple features with the same attribute you may have to create a filter or use rule based labeling so that the label only displays on selected features. Is this the case?
This used under Layer properties > Labels > Rendering > Show Label:
regexp_match( "ELEVATION" , '.*00$')
limits the labeling on a layer displaying topo lines to only 100M increments. The data in the attribute ELEVATION matches 700 or 2500 or anything ending in "00" Not sure what attribute on your dataset distinguishes one feature from another. Create one if there isn't one already. hope this helps
If there is no particular rule to which labels you want to hide and you want to manually toggle visibility, try playing around with the in-built label editing tools in QGIS 3.x (if you don't see it it's under View > Toolbars > Label Toolbar).
For multi-part features I have sometimes used the move label feature to reduce it to one label. You can reset the label to the default visibility by control-clicking and dragging your mouse across the area where the label used to be.
Note that the label positioning is saved within an auxiliary database in the qgis project and not your file itself.
For line features another thing you can try is under the label settings > Rendering (the paintbrush symbol at the end) check 'Merge connected lines to avoid duplicate labels'.